stp: Fix bpdu tx problem in listening state

stp: Fix bpdu tx problem in listening state

The restriction only allows to send bpdu in forwarding state in
compose_output_action__. But a port could send bpdu in listening
and learning state according to comments in lib/stp.h(State of
an STP port).

Until this commit, OVS did not send out BPDUs in listening and learning
states.  But those two states are temporary, the stp port will be in
forwarding state and send out BPDUs eventually (In the default
configuration listening and learning states last 15+15 second).  Therefore,
this bug increased convergence time but did not entirely break STP.

Signed-off-by: kmindg <kmindg@gmail.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>