xfs: move xfs_inode_ag_iterator to be closer to the perag walking code

Move the xfs_inode_ag_iterator function to be nearer xfs_inode_ag_walk
so that we don't have to scroll back and forth to figure out how the
incore inode walking function works.  No functional changes.

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