[B.A.T.M.A.N.] Basic Multicast Optimizations

["Linus Lüssing" <linus.luessing@web.de>]

This is the third revision of the basic multicast optimization patches.
It includes one functional and some compat fixes, and some style improvements,
thanks to Simons feedback:

PATCHv3 1/3:
* the pmc_rcu functions got removed - they weren't actually used in the
  submitted revision - so there's no more patch for net to export
  for_each_pmc_rcu() needed anymore. Should also fix Simon's compat error.
* Compat code for netdev_for_each_mc_addr() added.
  (I needed to introduce a batadv_hw_addr because I wasn't able to achieve
  backwards compatibility with a netdev_hw_addr - my "official reasoning" will
  be that netdev_hw_addr is unnecessarily bloated, struct batadv_hw_addr
  has just the right size to do the job)
* Compat fix for IFF_BRIDGE_PORT
* Removed primary_if->soft_iface in batadv_mcast_mla_tt_update() as we
  can always use bat_priv->soft_iface instead.

PATCHv3 2/3
* Renamed num_non_aware to a more specific num_no_mla - because we will need
  things like a num_no_tracker for instance with the multicast tracker feature
  in the future.
* Fixed removal of orig_node's without BATADV_MCAST_LISTENER_ANNOUNCEMENT
  flag, update num_no_mla properly now.

PATCHv3 3/3
* Renamed batadv_mcast_flood() to batadv_mcast_forw_mode() because it does not
  only show whether to flood or not (=drop), but also whether to forward via
  unicast. In the future with the multicast tracker feature a fourth return value
  is going to be added, making the name "batadv_mcast_flood()" even less fitting.
* Simon's style suggestions for batadv_mcast_forw_mode() and
  batadv_interface_tx().
* batadv_mcast_forw_mode() now returns an enum (thanks to Marek and Antonio for
  the suggestion).

I did not change:
> I don't quite understand why you return -1, maybe the packet could still
> be forwarded even if it could not be pulled?

Because if it returns -1 then something is wrong, for instance we could be
out of memory. If we are out of memory then we probably won't be able to
forward any packet. Also we shouldn't allocate any more memory for one thing
but I think it is also better to drop/free packets to save some memory
to increase the possibility of the system to recover without crashing.

Cheers, Linus