[BRIDGE]: if no STP then forward all BPDUs

If a bridge is not running STP, then it has no way to detect a cycle
in the network. But if it is not running STP and some other machine
or device is running STP, then if STP BPDU's get forwarded to it can
detect the cycle.

This is how the old 2.4 and early 2.6 code worked.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>