xfs: refactor iwalk code to handle walking inobt records

Refactor xfs_iwalk_ag_start and xfs_iwalk_ag so that the bits that are
particular to bulkstat (trimming the start irec, starting inode
readahead, and skipping empty groups) can be controlled via flags in the
iwag structure.

This enables us to add a new function to walk all inobt records which
will be used for the new INUMBERS implementation in the next patch.

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