netfilter: conntrack: unregister ipv4 sockopts on error unwind

When ipv6 sockopt register fails, the ipv4 one needs to be removed.

Fixes: a0ae2562 ("netfilter: conntrack: remove l3proto abstraction")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>