prevents a possible endless loop in pf_get_sport() with 'static-port'
ICMP state entries use the ICMP ID as port for the unique state key. When
checking for a usable key, construct the key in the same way. Otherwise,
a colliding key might be missed or a state insertion might be refused even
though it could be inserted. The second case triggers the endless loop,
possibly allowing a NATed LAN client to lock up the kernel.
PR: kern/74930
Reported and tested by: Hugo Silva, Srebrenko Sehic
MFC after: 3 days
2726bbe221