There was a corner case where linking the inp to the state key did

There was a corner case where linking the inp to the state key did
work in pf.  The function pf_inp_link() takes the state key from
the mbuf and not the one pf_find_state() has just found.  Introduce
a new function pf_state_key_link_inpcb() that links the given state
key and inpcb together with some sanity checks.
OK sashan@