netfilter: SYNPROXY target: restrict to INPUT/FORWARD

Fix a crash in synproxy_send_tcp() when using the SYNPROXY target in the
PREROUTING chain caused by missing routing information.

Reported-by: Nicki P. <xastx@gmx.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>