xfs: export _inobt_btrec_to_irec and _ialloc_cluster_alignment for scrub

Create a function to extract an in-core inobt record from a generic
btree_rec union so that scrub will be able to check inobt records
and check inode block alignment.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>