Btrfs: do not backup tree roots when fsync

It doesn't make sense to backup tree roots when doing fsync, since
during fsync those tree roots have not been consistent on disk.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: Qu Wenruo <quwenruo.btrfs@gmx.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -3641,7 +3641,14 @@ int write_all_supers(struct btrfs_fs_info *fs_info, int max_mirrors)
 	u64 flags;
 
 	do_barriers = !btrfs_test_opt(fs_info, NOBARRIER);
-	backup_super_roots(fs_info);
+
+	/*
+	 * max_mirrors == 0 indicates we're from commit_transaction,
+	 * not from fsync where the tree roots in fs_info have not
+	 * been consistent on disk.
+	 */
+	if (max_mirrors == 0)
+		backup_super_roots(fs_info);
 
 	sb = fs_info->super_for_commit;
 	dev_item = &sb->dev_item;