IPoIB: Set pkt_type correctly for multicast packets (fix IGMP breakage)

IGMP processing is broken because the IPOIB does not set the
skb->pkt_type the right way for multicast traffic.  All incoming
packets are set to PACKET_HOST which means that igmp_recv() will
ignore the IGMP broadcasts/multicasts.

This in turn means that the IGMP timers are firing and are sending
information about multicast subscriptions unnecessarily.  In a large
private network this can cause traffic spikes.

Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>