pf: Improve route-to handling of pfsync'd states

When a state if pfsync’d to a different host it doesn’t get all of the
expected pointers, including the pointer to the struct pfi_kif / struct
ifnet rt_kif pointer. (I.e. the interface to route out on).

That in turn means that pf_route() ends up dropping the packet.

Use the rule's struct pfi_kif pointer so we can still route out of the
expected interface.

MFC after: 2 weeks
Sponsored by: Orange Business Services