The main difference is that ping now reports icmp_seq instead of
icmp_req, so we match for icmp_.eq, which works with both releases.
tcpdump now also reports port 4500 as ipsec-nat-t.
As we now reuse the reqid for identical SAs, the behavior changes for
transport connections to multiple peers behind the same NAT. Instead of
rejecting the SA, we now have two valid SAs active. For the reverse path,
however, sun sends traffic always over the newer SA, resembling the behavior
before we introduced explicit SA conflicts for different reqids.