IPoIB: Only set Q_Key once: after joining broadcast group

The current code will set the Q_Key for any join of a non-sendonly
multicast group.  The operation involves a modify QP operation, which
is fairly heavyweight, and is only really required after the join of
the broadcast group.  Fix this by adding a parameter to ipoib_mcast_attach()
to control when the Q_Key is set.

Signed-off-by: Eli Cohen <eli@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>