[NETFILTER] NAT: Fix module refcount dropping too far

The unknown protocol is used as a fallback when a protocol isn't known.
Hence we cannot handle it failing, so don't set ".me".  It's OK, since we
only grab a reference from within the same module (iptable_nat.ko), so we
never take the module refcount from 0 to 1.

Also, remove the "protocol is NULL" test: it's never NULL.
Signed-off-by: Rusty Rusty <rusty@rustcorp.com.au>
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>

net/ipv4/netfilter/ip_nat_core.c

@@ -66,10 +66,8 @@ ip_nat_proto_find_get(u_int8_t protonum)
 	 * removed until we've grabbed the reference */
 	preempt_disable();
 	p = __ip_nat_proto_find(protonum);
-	if (p) {
 	if (!try_module_get(p->me))
 		p = &ip_nat_unknown_protocol;
-	}
 	preempt_enable();
 
 	return p;

net/ipv4/netfilter/ip_nat_proto_unknown.c

@@ -62,7 +62,7 @@ unknown_print_range(char *buffer, const struct ip_nat_range *range)
 
 struct ip_nat_protocol ip_nat_unknown_protocol = {
 	.name			= "unknown",
-	.me			= THIS_MODULE,
+	/* .me isn't set: getting a ref to this cannot fail. */
 	.manip_pkt		= unknown_manip_pkt,
 	.in_range		= unknown_in_range,
 	.unique_tuple		= unknown_unique_tuple,