netfilter: xt_connmark: fix list corruption on rmmod

This needs to use xt_unregister_targets, else new revision is left
on the list which then causes list to point to a target struct that has been free'd.

Fixes: 472a73e0

 ("netfilter: xt_conntrack: Support bit-shifting for CONNMARK & MARK targets.")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>