xfs: rename the m_writeio_* fields in struct xfs_mount

Use the allocsize name to match the mount option and usage instead.

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>