mac80211: Fix mesh portal communication with other mesh nodes.

Fixed a bug where if a mesh interface has a different MAC address from its bridge
interface, then it would not be able to send data traffic to any other mesh node.
This also adds support for communication between mesh nodes and external bridged
nodes by using a 6 address format if the source is a node within the mesh and the
destination is an external node proxied by a mesh portal.

Signed-off-by: Joel A Fernandes <agnel.joel@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>