mac80211: implement support for 4-address frames for AP and client mode

In some situations it might be useful to run a network with an
Access Point and multiple clients, but with each client bridged
to a network behind it. For this to work, both the client and the
AP need to transmit 4-address frames, containing both source and
destination MAC addresses.
With this patch, you can configure a client to communicate using
only 4-address frames for data traffic.
On the AP side you can enable 4-address frames for individual
clients by isolating them in separate AP VLANs which are configured
in 4-address mode.
Such an AP VLAN will be limited to one client only, and this client
will be used as the destination for all traffic on its interface,
regardless of the destination MAC address in the packet headers.
The advantage of this mode compared to regular WDS mode is that it's
easier to configure and does not require a static list of peer MAC
addresses on any side.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>