bridge: relay bridge multicast pkgs if !STP

Currently the bridge catches all STP packets; even if STP is turned
off.  This prevents other systems (which do have STP turned on)
from being able to detect loops in the network.

With this patch, if STP is off, then any packet sent to the STP
multicast group address is forwarded to all ports.

Based on earlier patch by Joakim Tjernlund with changes
to go through forwarding (not local chain), and optimization
that only last octet needs to be checked.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>