pf_state_key_link_reverse() is prone to race on parallel forwarding

pf_state_key_link_reverse() is prone to race on parallel forwarding

we need to adjust assertions. at time we call pf_state_key_link_reverse()
is state_key either linked to correct reverse peer or not linked at all.
The pf_state_key_link_reverse() is being called as a reader ons tate_lock.
There might be more packets, which try to update the state key.

OK bluhm@