Some EoIB implementations utilise an EoIB-to-Ethernet gateway device
that does not perform a FullMember join to the multicast group for the
EoIB broadcast domain.  This has various exciting side-effects, such
as requiring every EoIB node to send every broadcast packet twice.

As an added bonus, the gateway may also break the EoIB MAC address to
GID mapping protocol by sending Ethernet-sourced packets from the
wrong QPN.

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