ztest: display non-index properties properly at verbose level 6

At verbosity levels of 6 or greater, ztest_dsl_prop_set_uint64() attempts
to display the value of all properties as indexed values regardless of
whether the property is an indexed value or simply an un-indexed integer.
This patch causes the numeric value of the property to be displayed if
zfs_prop_index_to_string() fails.

Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3649

cmd/ztest/ztest.c

@@ -1097,9 +1097,16 @@ ztest_dsl_prop_set_uint64(char *osname, zfs_prop_t prop, uint64_t value,
 	VERIFY0(dsl_prop_get_integer(osname, propname, &curval, setpoint));
 
 	if (ztest_opts.zo_verbose >= 6) {
-		VERIFY(zfs_prop_index_to_string(prop, curval, &valname) == 0);
-		(void) printf("%s %s = %s at '%s'\n",
-		    osname, propname, valname, setpoint);
+		int err;
+
+		err = zfs_prop_index_to_string(prop, curval, &valname);
+		if (err)
+			(void) printf("%s %s = %llu at '%s'\n",
+			    osname, propname, (unsigned long long)curval,
+				setpoint);
+		else
+			(void) printf("%s %s = %s at '%s'\n",
+			    osname, propname, valname, setpoint);
 	}
 	umem_free(setpoint, MAXPATHLEN);