Remove the GRAB_OUR_PACKETS() macro and sprinkle its remains over
bridge_input():
- check much earlier if we should punt the packet to vlan(4), or if it's filtered by pfil, or if we need to learn the address.
- actual local packet handling is moved to a new function, bridge_try_local().
While here, change handling of frames received from discarding (STP)
ports to drop the frames instead of passing them to the host IP stack.