lock on the INP before calling the tunnel protocol, else a LOR
may occur (it does with SCTP for sure). Instead we must acquire a
ref count and release the lock, taking care to allow for the case
where the UDP socket has gone away and *not* unlocking since the
refcnt decrement on the inp will do the unlock in that case.
Reviewed by: tuexen
MFC after: 3 weeks
c0d1be08f6