bfd: Require bfd control packet received in forwarding_if_rx mode.

This commit adds a requirement that bfd session must receive at least
one bfd control packet every 100 * bfd->cfg_min_rx amount of time in
forwarding_if_rx mode.  Otherwise, even if the data packets are received
on the monitored interface, the bfd->forwarding is still false.

Since the datapath flow is not purged when the userspace Open Vswitch
crashes, data packet can still be forwarded through the tunnel and
fool the remote BFD session in forwarding_if_rx mode.  Thus, this commit
can prevent the remote BFD session from falsely declaring tunnel liveness
in this situation.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>