netfilter: ctnetlink: avoid null pointer dereference

Dan Carpenter points out that deref occurs after NULL check, we should
re-fetch the pointer and check that instead.

Fixes: 2c205dd3 ("netfilter: add struct nf_nat_hook and use it")
Reported-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_netlink.c

@@ -1446,7 +1446,8 @@ ctnetlink_parse_nat_setup(struct nf_conn *ct,
 		}
 		nfnl_lock(NFNL_SUBSYS_CTNETLINK);
 		rcu_read_lock();
-		if (nat_hook->parse_nat_setup)
+		nat_hook = rcu_dereference(nf_nat_hook);
+		if (nat_hook)
 			return -EAGAIN;
 #endif
 		return -EOPNOTSUPP;