xfs: inode btree scrubber should calculate im_boffset correctly

The im_boffset field is in units of bytes, whereas XFS_INO_OFFSET
returns a value in units of inodes.  Convert the units so that scrub on
a 64k-block filesystem works correctly.

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