netfilter: synproxy: fix conntrackd interaction

This patch fixes the creation of connection tracking entry from
netlink when synproxy is used. It was missing the addition of
the synproxy extension.

This was causing kernel crashes when a conntrack entry created by
conntrackd was used after the switch of traffic from active node
to the passive node.

Signed-off-by: Eric Leblond <eric@regit.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>