btrfs: sync ioctl, handle errors after transaction start

The version merged to 3.19 did not handle errors from start_trancaction
and could pass an invalid pointer to commit_transaction.

Fixes: 6b5fe46d ("btrfs: do commit in sync_fs if there are pending changes")
Reported-by: NFilipe Manana <fdmanana@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.cz>
Signed-off-by: NChris Mason <clm@fb.com>

fs/btrfs/super.c

@@ -1001,9 +1001,9 @@ int btrfs_sync_fs(struct super_block *sb, int wait)
 			if (fs_info->pending_changes == 0)
 				return 0;
 			trans = btrfs_start_transaction(root, 0);
-		} else {
-			return PTR_ERR(trans);
 		}
+		if (IS_ERR(trans))
+			return PTR_ERR(trans);
 	}
 	return btrfs_commit_transaction(trans, root);
 }