Btrfs: don't bother kicking async if there's nothing to reclaim

We do this check when we start the async reclaimer thread, might as well check
before we kick it off to save us some cycles.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.c

@@ -4944,6 +4944,9 @@ static inline int need_do_async_reclaim(struct btrfs_space_info *space_info,
 	if ((space_info->bytes_used + space_info->bytes_reserved) >= thresh)
 		return 0;
 
+	if (!btrfs_calc_reclaim_metadata_size(fs_info->fs_root, space_info))
+		return 0;
+
 	return (used >= thresh && !btrfs_fs_closing(fs_info) &&
 		!test_bit(BTRFS_FS_STATE_REMOUNTING, &fs_info->fs_state));
 }