bfd: Change the update of forwarding flag.

bfd: Change the update of forwarding flag.

Currently, we update the forwarding flag in bfd_set_state() and in
bfd_forwarding_if_rx_update() if bfd_forwarding_if_rx is enabled.
However, these are not the exact places where the forwarding flag
needs to be updated.  The exact places are in the bfd_process_packet()
where bfd status are changed based on received control packet, and in
the flow_push_stats() and compose_output_action__() where the
rx_packet counter is updated.

This commit changes the update of forwarding flag to the places
mentioned above.

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