Correct refcount_add in dmu_zfetch

refcount_add_many(foo,N) is not the same as
for (i=0; i < N; i++) { refcount_add(foo); }

Unfortunately, this is only actually true with debug kernels and
reference_tracking_enable=1.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12589 
Closes #12602

include/sys/zfs_refcount.h

@@ -72,6 +72,14 @@ int zfs_refcount_is_zero(zfs_refcount_t *);
 int64_t zfs_refcount_count(zfs_refcount_t *);
 int64_t zfs_refcount_add(zfs_refcount_t *, const void *);
 int64_t zfs_refcount_remove(zfs_refcount_t *, const void *);
+/*
+ * Note that (add|remove)_many add/remove one reference with "number" N,
+ * _not_ make N references with "number" 1, which is what vanilla
+ * zfs_refcount_(add|remove) would do if called N times.
+ *
+ * Attempting to remove a reference with number N when none exists is a
+ * panic on debug kernels with reference_tracking enabled.
+ */
 int64_t zfs_refcount_add_many(zfs_refcount_t *, uint64_t, const void *);
 int64_t zfs_refcount_remove_many(zfs_refcount_t *, uint64_t, const void *);
 void zfs_refcount_transfer(zfs_refcount_t *, zfs_refcount_t *);

module/zfs/dmu_zfetch.c

@@ -488,7 +488,8 @@ dmu_zfetch_run(zstream_t *zs, boolean_t missed, boolean_t have_lock)
 	issued = pf_end - pf_start + ipf_end - ipf_start;
 	if (issued > 1) {
 		/* More references on top of taken in dmu_zfetch_prepare(). */
-		zfs_refcount_add_many(&zs->zs_refs, issued - 1, NULL);
+		for (int i = 0; i < issued - 1; i++)
+			zfs_refcount_add(&zs->zs_refs, NULL);
 	} else if (issued == 0) {
 		/* Some other thread has done our work, so drop the ref. */
 		if (zfs_refcount_remove(&zs->zs_refs, NULL) == 0)