netlink: compare policy more accurately

The maxtype is really an integral part of the policy, and while we
haven't gotten into a situation yet where this happens, it seems
that some developer might eventually have two places pointing to
identical policies, with different maxattr to exclude some attrs
in one of the places.

Even if not, it's really the right thing to compare both since the
two data items fundamentally belong together.

v2:
 - also do the proper comparison in get_policy_idx()

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>