resolved: ignore mDNS multicast packets originated from our host

By default all sent multicast packets are looped back to the
sending host. And since the code that adds the mDNS socket to
the mDNS multicast group is the same as for LLMNR I decided
that for now it'd be safer to filter looped back packets in the
application than to switch loopback off. Because I don't know
what would be the implications for LLMNR with loopback switched off.

I suspect that loopback can be disabled for LLMNR safely too and the
function manager_our_packet() is not really needed. But I'd
prefer to do it in another patchset.

Signed-off-by: Dmitry Rozhkov <dmitry.rozhkov@linux.intel.com>

src/resolve/resolved-mdns.c

@@ -77,6 +77,9 @@ static int on_mdns_packet(sd_event_source *s, int fd, uint32_t revents, void *us
         if (r <= 0)
                 return r;
 
+        if (manager_our_packet(m, p))
+                return 0;
+
         scope = manager_find_scope(m, p);
         if (!scope) {
                 log_warning("Got mDNS UDP packet on unknown scope. Ignoring.");