xfs: remove custom do_div implementations

Long ago, all this gunk was added with a lament about problems
with gcc's do_div, and a fun recommendation in the changelog:

 egcs-2.91.66 is the recommended compiler version for building XFS.

All this special stuff was needed to work around an old gcc bug,
apparently, and it's been there ever since.

There should be no need for this anymore, so remove it.

Remove the special 32-bit xfs_do_mod as well; just let the
kernel's do_div() handle all this.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-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>