IPoIB: Yet another fix for send-only joins

Even after the last fix, it's still possible for a send-only join to
start before the join for the broadcast group has finished.  This
could cause us to create a multicast group using attributes from the
broadcast group that haven't been initialized yet, so we would use
garbage for the Q_Key, etc.  Fix this by waiting until the broadcast
group's attached flag is set before starting send-only joins.

Signed-off-by: Roland Dreier <rolandd@cisco.com>