[ipoib] Resimplify test for received broadcast packets

Commit e62e52b ("[ipoib] Simplify test for received broadcast
packets") relies upon the multicast LID being present in the
destination address vector as passed to ipoib_complete_recv().
Unfortunately, this information is not present in many Infiniband
devices' completion queue entries.

Fix by testing instead for the presence of a multicast GID.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/ipoib.c

 	ethhdr->h_protocol = net_proto;
 
 	/* Construct destination address */
-	if ( IB_LID_MULTICAST ( dest->lid ) ) {
-		/* Multicast LID; use the Ethernet broadcast address */
+	if ( dest->gid_present && IB_GID_MULTICAST ( &dest->gid ) ) {
+		/* Multicast GID: use the Ethernet broadcast address */
 		memcpy ( &ethhdr->h_dest, eth_broadcast,
 			 sizeof ( ethhdr->h_dest ) );
 	} else {

src/include/ipxe/ib_packet.h

 #define IB_GID_ARGS( gid ) \
 	IB_GUID_ARGS ( &(gid)->s.prefix ), IB_GUID_ARGS ( &(gid)->s.guid )
 
+/** Test for multicast GID */
+#define IB_GID_MULTICAST( gid ) ( (gid)->bytes[0] == 0xff )
+
 /** An Infiniband Local Route Header */
 struct ib_local_route_header {
 	/** Virtual lane and link version */