xfs: convert xfs_alloc_vextent_iterate_ags() to use perag walker

Now that the AG iteration code in the core allocation code has been
cleaned up, we can easily convert it to use a for_each_perag..()
variant to use active references and skip AGs that it can't get
active references on.

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