Add new helpers to convert units of fs blocks into inodes, and AG blocks
into AG inodes, respectively.  Convert all the open-coded conversions
and XFS_OFFBNO_TO_AGINO(, , 0) calls to use them, as appropriate.  The
OFFBNO_TO_AGINO macro is retained for xfs_repair.
Signed-off-by: NDarrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: NBrian Foster <bfoster@redhat.com>