IPoIB: Lock accesses to multicast packet queues

Avoid corrupting mcast->pkt_queue by serializing access with
priv->tx_lock.  Also, update dropped packet statistics to count
multicast packets removed from pkt_queue as dropped.

Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>