mrib: receive_mrt: workaround for RTL8373 queries

Fixes the following warning caused by RTL8373 sending IGMP queries without
including the mandatory IPv4 Router Alert option:
mrib_receive_mrt: ignoring invalid IGMP-message of type 11 from <ip> on <ifindex>

According to RFC3376 9.1 [1]:
- Hosts SHOULD ignore v2 or v3 Queries without the Router-Alert option.
However, we can add an exception to be compatible with non-compliant switches.

[1] https://datatracker.ietf.org/doc/html/rfc3376#section-9.1

Link: #23
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>

Author: Noltari

src/mrib.c

@@ -306,7 +306,7 @@ static void mrib_receive_mrt(struct uloop_fd *fd, __unused unsigned flags)
 			uint32_t *opts = (uint32_t*)&iph[1];
 			bool alert = (void*)&opts[1] <= (void*)igmp && *opts == ipv4_rtr_alert;
 			if (!alert && (igmp->type != IGMP_HOST_MEMBERSHIP_QUERY ||
-							(size_t)len > sizeof(*igmp) || igmp->code > 0)) {
+							(size_t)len > sizeof(*igmp))) {
 				L_WARN("%s: ignoring invalid IGMP-message of type %x from %s on %d",
 						__FUNCTION__, igmp->type, addrbuf, ifindex);
 				continue;