sd-ndisc: Reset counter for sent Router Solicitations (#5874)

Reset also the counter for number of Router Solicitations sent when
the associated file descriptor is closed and the event source
unreferenced. With this change the router discovery can now be
stopped and restarted arbitrary many times.

src/libsystemd-network/sd-ndisc.c

@@ -131,6 +131,7 @@ static int ndisc_reset(sd_ndisc *nd) {
         nd->timeout_event_source = sd_event_source_unref(nd->timeout_event_source);
         nd->recv_event_source = sd_event_source_unref(nd->recv_event_source);
         nd->fd = safe_close(nd->fd);
+        nd->nd_sent = 0;
 
         return 0;
 }