netfilter: ctnetlink: Add missing annotation for ctnetlink_parse_nat_setup()

Sparse reports a warning at ctnetlink_parse_nat_setup()

warning: context imbalance in ctnetlink_parse_nat_setup()
	- unexpected unlock

The root cause is the missing annotation at ctnetlink_parse_nat_setup()
Add the missing __must_hold(RCU) annotation

Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_netlink.c

@@ -1533,6 +1533,7 @@ static int
 ctnetlink_parse_nat_setup(struct nf_conn *ct,
 			  enum nf_nat_manip_type manip,
 			  const struct nlattr *attr)
+	__must_hold(RCU)
 {
 	struct nf_nat_hook *nat_hook;
 	int err;