xfs: move bulkstat ichunk helpers to iwalk code

Now that we've reworked the bulkstat code to use iwalk, we can move the
old bulkstat ichunk helpers to xfs_iwalk.c.  No functional changes here.

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