ofproto-dpif: Segregate CFM, LACP, and STP traffic into separate queues.

ofproto-dpif: Segregate CFM, LACP, and STP traffic into separate queues.

Until now, packets for these special protocols have been mixed with general
traffic in the kernel-to-userspace queues.  This means that a big-enough
storm of new flows in these queues can cause packets for these special
protocols to be dropped at this interface, fooling userspace into believing
that, say, no CFM packets have been received even though they are arriving
at the expected rate.

This commit moves special protocols to a dedicated kernel-to-userspace
queue to avoid the problem.

Bug #7550.
Signed-off-by: Ben Pfaff <blp@nicira.com>