xfs: rearrange xfs_inode_walk_ag parameters

The perag structure already has a pointer to the xfs_mount, so we don't
need to pass that separately and can drop it.  Having done that, move
iter_flags so that the argument order is the same between xfs_inode_walk
and xfs_inode_walk_ag.  The latter will make things less confusing for a
future patch that enables background scanning work to be done in
parallel.

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