IB/ipoib: deserialize multicast joins

Allow the ipoib layer to attempt to join all outstanding multicast
groups at once.  The ib_sa layer will serialize multiple attempts to
join the same group, but will process attempts to join different groups
in parallel.  Take advantage of that.

In order to make this happen, change the mcast_join_thread to loop
through all needed joins, sending a join request for each one that we
still need to join.  There are a few special cases we handle though:

1) Don't attempt to join anything but the broadcast group until the join
of the broadcast group has succeeded.
2) No longer restart the join task at the end of completion handling.
If we completed successfully, we are done.  The join task now needs kicked
either by mcast_send or mcast_restart_task or mcast_start_thread, but
should not need started anytime else except when scheduling a backoff
attempt to rejoin.
3) No longer use separate join/completion routines for regular and
sendonly joins, pass them all through the same routine and just do the
right thing based on the SENDONLY join flag.
4) Only try to join a SENDONLY join twice, then drop the packets and
quit trying.  We leave the mcast group in the list so that if we get a
new packet, all that we have to do is queue up the packet and restart
the join task and it will automatically try to join twice and then
either send or flush the queue again.
Signed-off-by: Doug Ledford <dledford@redhat.com>