IB/ipoib: increase the max mcast backlog queue

When performing sendonly joins, we queue the packets that trigger
the join until the join completes.  This may take on the order of
hundreds of milliseconds.  It is easy to have many more than three
packets come in during that time.  Expand the maximum queue depth
in order to try and prevent dropped packets during the time it
takes to join the multicast group.

Signed-off-by: Doug Ledford <dledford@redhat.com>