xfs: return errors in xfs_fs_sync_fs

Now that the VFS will do something with the return values from
->sync_fs, make ours pass on error codes.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Christian Brauner <brauner@kernel.org>