[ipoib] Transmit multicast packets as broadcasts

[ipoib] Transmit multicast packets as broadcasts

Multicast MAC addresses will never have REMAC cache entries, and the
corresponding multicast IPoIB MAC address cannot be obtained simply by
issuing an ARP request.

For the trivial volume of multicast packets that we expect to send in
any realistic scenario, the simplest solution is to send them as
broadcasts instead.

Reported-by: Wissam Shoukair <wissams@mellanox.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>