xfs: convert raw ag walks to use for_each_perag

Convert the raw walks to an iterator, pulling the current AG out of
pag->pag_agno instead of the loop iterator variable.

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