dmu_zfetch: don't leak unreferenced stream when zfetch is freed

Currently streams are only freed when:
  - They have no referencing zfetch and and their I/O references
    go to zero.
  - They are more than 2s old and a new I/O request comes in on
    the same zfetch.

This means that we will leak unreferenced streams when their zfetch
structure is freed.

This change checks the reference count on a stream at zfetch free
time. If it is zero we free it immediately. If it has remaining
references we allow the prefetch callback to free it at I/O
completion time.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Adam Moss <[email protected]>
Reviewed-by: Ryan Moeller <[email protected]>
Signed-off-by: Matt Macy <[email protected]>
Closes openzfs#11052

module/zfs/dmu_zfetch.c

@@ -161,8 +161,12 @@ dmu_zfetch_fini(zfetch_t *zf)
 	zstream_t *zs;
 
 	mutex_enter(&zf->zf_lock);
-	while ((zs = list_head(&zf->zf_stream)) != NULL)
-		dmu_zfetch_stream_orphan(zf, zs);
+	while ((zs = list_head(&zf->zf_stream)) != NULL) {
+		if (zfs_refcount_count(&zs->zs_blocks) != 0)
+			dmu_zfetch_stream_orphan(zf, zs);
+		else
+			dmu_zfetch_stream_remove(zf, zs);
+	}
 	mutex_exit(&zf->zf_lock);
 	list_destroy(&zf->zf_stream);
 	mutex_destroy(&zf->zf_lock);