xfs: convert xfs_iwalk to use perag references

Rather than manually walking the ags and passing agnunbers around,
pass the perag for the AG we are currently working on around in the
iwalk structure.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>