mac80211: multicast to unicast conversion

mac80211: multicast to unicast conversion

Add the ability for an AP (and associated VLANs) to perform
multicast-to-unicast conversion for ARP, IPv4 and IPv6 frames
(possibly within 802.1Q). If enabled, such frames are to be sent
to each station separately, with the DA replaced by their own
MAC address rather than the group address.

Note that this may break certain expectations of the receiver,
such as the ability to drop unicast IP packets received within
multicast L2 frames, or the ability to not send ICMP destination
unreachable messages for packets received in L2 multicast (which
is required, but the receiver can't tell the difference if this
new option is enabled.)

This also doesn't implement the 802.11 DMS (directed multicast
service).

Signed-off-by: Michael Braun <michael-dev@fami-braun.de>
[use true/false, rename label to the correct "multicast",
 use __be16 for ethertype and network order for constants]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>