netfilter: synproxy: fix manual bump of the reference counter

This operation is handled by nf_synproxy_ipv4_init() now.

Fixes: d7f9b2f1 ("netfilter: synproxy: extract SYNPROXY infrastructure from {ipt, ip6t}_SYNPROXY")
Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>