conntrack: RFC5961 challenge ACK confuse conntrack LAST-ACK transition

In compliance with RFC5961, the network stack send challenge ACK in
response to spurious SYN packets, since commit 0c228e83 ("tcp:
Restore RFC5961-compliant behavior for SYN packets").

This pose a problem for netfilter conntrack in state LAST_ACK, because
this challenge ACK is (falsely) seen as ACKing last FIN, causing a
false state transition (into TIME_WAIT).

The challenge ACK is hard to distinguish from real last ACK.  Thus,
solution introduce a flag that tracks the potential for seeing a
challenge ACK, in case a SYN packet is let through and current state
is LAST_ACK.

When conntrack transition LAST_ACK to TIME_WAIT happens, this flag is
used for determining if we are expecting a challenge ACK.

Scapy based reproducer script avail here:
 https://github.com/netoptimizer/network-testing/blob/master/scapy/tcp_hacks_3WHS_LAST_ACK.py

Fixes: 0c228e83

 ("tcp: Restore RFC5961-compliant behavior for SYN packets")
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>