IPoIB: Wait for join to finish before freeing mcast struct

ipoib_mcast_restart_task() might free an mcast object while a join
request is still outstanding, leading to an oops when the query
completes.  Fix this by waiting for query to complete, similar to what
ipoib_stop_thread() is doing.  The wait for mcast completion code is
consolidated in wait_for_mcast_join().

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