[NETFILTER] nf_conntrack: Add missing code to TCP conntrack module

Looks like the nf_conntrack TCP code was slightly mismerged: it does
not contain an else branch present in the IPv4 version. Let's add that
code and make the testsuite happy.

Signed-off-by: KOVACS Krisztian <hidden@balabit.hu>
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>