xfs: pack holes in xfs_defer_ops and xfs_trans

Both structures have holes due to member alignment. Move dop_low to
the end of xfs_defer ops to sanitize the cache line alignment and
move t_flags to save 8 bytes in xfs_trans.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bill O'Donnell <billodo@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>