xfs: fold xfs_buf_ioend_finish into xfs_ioend

No need to keep a separate helper for this logic.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>