hv_netvsc: enable multicast if necessary

My recent change to netvsc drive in how receive flags are handled
broke multicast.  The Hyper-v/Azure virtual interface there is not a
multicast filter list, filtering is only all or none. The driver must
enable all multicast if any multicast address is present.

Fixes: 009f766ca238 ("hv_netvsc: filter multicast/broadcast")
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/hyperv/rndis_filter.c

@@ -858,7 +858,7 @@ static void rndis_set_multicast(struct work_struct *w)
 	if (flags & IFF_PROMISC) {
 		filter = NDIS_PACKET_TYPE_PROMISCUOUS;
 	} else {
-		if (flags & IFF_ALLMULTI)
+		if (!netdev_mc_empty(rdev->ndev) || (flags & IFF_ALLMULTI))
 			filter |= NDIS_PACKET_TYPE_ALL_MULTICAST;
 		if (flags & IFF_BROADCAST)
 			filter |= NDIS_PACKET_TYPE_BROADCAST;