common/dns_resolve: free res_ninit/res_nquery

When sanitizer is enabled, unittest_osdscrub shows

```
=================================================================
==1633952==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 28 byte(s) in 1 object(s) allocated from:
    #0 0xaaaab4e108e0 in malloc (/root/ceph/build/bin/unittest_osdscrub+0x1ed08e0) (BuildId: b3cfa2137be96d75535beecf0f2500cec10c7550)
    ceph#1 0xffffa8cac2f8 in __res_context_send resolv/./resolv/res_send.c:334:9
    ceph#2 0xffffa8ca9c54 in __res_context_query resolv/./resolv/res_query.c:216:6
    ceph#3 0xffffa8caa4a8 in __res_context_querydomain resolv/./resolv/res_query.c:625:9
    ceph#4 0xffffa8caa4a8 in __res_context_search resolv/./resolv/res_query.c:381:9
    ceph#5 0xffffa8caaa20 in context_search_common resolv/./resolv/res_query.c:550:16
    ceph#6 0xffffa8caaa20 in res_nsearch resolv/./resolv/res_query.c:563:10
    ceph#7 0xffffabbf1f64 in ceph::ResolvHWrapper::res_nsearch(__res_state*, char const*, int, int, unsigned char*, int) /root/ceph/src/common/dns_resolve.cc:37:10
    ceph#8 0xffffabbf6574 in ceph::DNSResolver::resolve_srv_hosts(ceph::common::CephContext*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, ceph::DNSResolver::SRV_Protocol, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::map<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, ceph::DNSResolver::Record, std::less<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, ceph::DNSResolver::Record> > >*) /root/ceph/src/common/dns_resolve.cc:295:19
    ceph#9 0xffffac8edaf0 in MonMap::init_with_dns_srv(ceph::common::CephContext*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, std::ostream&) /root/ceph/src/mon/MonMap.cc:935:36
    ceph#10 0xffffac8eeec8 in MonMap::build_initial(ceph::common::CephContext*, bool, std::ostream&) /root/ceph/src/mon/MonMap.cc:1014:20
    ceph#11 0xffffac85beb0 in MonClient::build_initial_monmap() /root/ceph/src/mon/MonClient.cc:93:18
    ceph#12 0xaaaab4e50d98 in TestOSDScrub_scrub_time_permit_Test::TestBody() /root/ceph/src/test/osd/TestOSDScrub.cc:73:6
    ceph#13 0xaaaab4f655b0 in void testing::internal::HandleSehExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) /root/ceph/src/googletest/googletest/src/gtest.cc:2605:10
    ceph#14 0xaaaab4f16264 in void testing::internal::HandleExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) /root/ceph/src/googletest/googletest/src/gtest.cc:2641:14
    ceph#15 0xaaaab4ec6ca8 in testing::Test::Run() /root/ceph/src/googletest/googletest/src/gtest.cc:2680:5
    ceph#16 0xaaaab4ec8bec in testing::TestInfo::Run() /root/ceph/src/googletest/googletest/src/gtest.cc:2858:11
    ceph#17 0xaaaab4eca1ec in testing::TestSuite::Run() /root/ceph/src/googletest/googletest/src/gtest.cc:3012:28
    ceph#18 0xaaaab4ee5fb0 in testing::internal::UnitTestImpl::RunAllTests() /root/ceph/src/googletest/googletest/src/gtest.cc:5723:44
    ceph#19 0xaaaab4f6f4c4 in bool testing::internal::HandleSehExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool>(testing::internal::UnitTestImpl*, bool (testing::internal::UnitTestImpl::*)(), char const*) /root/ceph/src/googletest/googletest/src/gtest.cc:2605:10
    ceph#20 0xaaaab4f1d4bc in bool testing::internal::HandleExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool>(testing::internal::UnitTestImpl*, bool (testing::internal::UnitTestImpl::*)(), char const*) /root/ceph/src/googletest/googletest/src/gtest.cc:2641:14
    ceph#21 0xaaaab4ee5428 in testing::UnitTest::Run() /root/ceph/src/googletest/googletest/src/gtest.cc:5306:10
    ceph#22 0xaaaab4e4b790 in RUN_ALL_TESTS() /root/ceph/src/googletest/googletest/include/gtest/gtest.h:2486:46
    ceph#23 0xaaaab4e49dbc in main /root/ceph/src/test/unit.cc:45:10
    ceph#24 0xffffa8bc73f8 in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16
    ceph#25 0xffffa8bc74c8 in __libc_start_main csu/../csu/libc-start.c:392:3
    ceph#26 0xaaaab4d9972c in _start (/root/ceph/build/bin/unittest_osdscrub+0x1e5972c) (BuildId: b3cfa2137be96d75535beecf0f2500cec10c7550)

-----------------------------------------------------
Suppressions used:
  count      bytes template
      1         45 ^MallocExtension::Initialize
-----------------------------------------------------

SUMMARY: AddressSanitizer: 28 byte(s) leaked in 1 allocation(s).
```

1. 'res_ninit/res_nquery' memory should be freed.

Signed-off-by: Rongqi Sun <sunrongqi@huawei.com>

src/common/dns_resolve.cc

@@ -56,6 +56,7 @@ DNSResolver::~DNSResolver()
 #ifdef HAVE_RES_NQUERY
   for (auto iter = states.begin(); iter != states.end(); ++iter) {
     struct __res_state *s = *iter;
+    res_nclose(s);
     delete s;
   }
 #endif