When processing a PREQ or PREP it's critical to use the incoming SN. If
that is improperly done routing loops and other types of badness can
happen. But the code was always processing path messages for deactivated
paths. This path fixes that so that if we have a valid SN then we use it
to verify that it is a message we can accept. For reference the relevant
section of the standard is 13.10.8.4 which doesn't address the deactivated
path case at all.

I also included a special case for when our peer reboots or restarts
networking. This is an important case because without it there can be a
very long delay before we accept path messages from that peer. It's also a
simple case and intimately associated with processing messages for
deactivated paths so I used one patch instead of two.
Signed-off-by: Alexis Green <agreen@cococorp.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>