IB/ipath: Discard multicast packets without a GRH

This patch fixes a bug where multicast packets without a GRH were not
being dropped as per the IB spec.

Signed-off-by: Ralph Campbell <ralph.campbell@qlogic.com>
Signed-off-by: Bryan O'Sullivan <bryan.osullivan@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>