resolve/mdns: silently ignore Known-Answers only packets

(cherry picked from commit dd51c43)
(cherry picked from commit c4f6807)

Author: yuwata

src/resolve/resolved-mdns.c

@@ -254,8 +254,9 @@ static int mdns_scope_process_query(DnsScope *s, DnsPacket *p) {
         if (r < 0)
                 return log_debug_errno(r, "Failed to extract resource records from incoming packet: %m");
 
+        /* TODO: Support Known-Answers only packets gracefully. */
         if (dns_question_size(p->question) <= 0)
-                return log_debug_errno(SYNTHETIC_ERRNO(EBADMSG), "Received mDNS query without question, ignoring.");
+                return 0;
 
         unicast_reply = mdns_should_reply_using_unicast(p);
         if (unicast_reply && !sender_on_local_subnet(s, p)) {