mac80211: implement multicast forwarding on fast-RX path

In AP (or VLAN) mode, when unicast 802.11 packets are received,
they might actually be multicast after conversion. In this case
the fast-RX path didn't handle them properly to send them back
to the wireless medium. Implement that by copying the SKB and
sending it back out.

The possible alternative would be to just punt the packet back
to the regular (slow) RX path, but since we have almost all of
the required code here already it's not so complicated to add
here. Punting it back would also mean acquiring the spinlock,
which would be bad for the stated purpose of the fast-RX path,
to enable well-performing parallel RX.

Cc: stable@vger.kernel.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>