_lock_irqsave() must be used to take it.
n_mcast_grps_lock is only taken from process context, so at first it
seems safe to take it with plain spin_lock(); however, it also nests
inside mcast_lock, and hence we could deadlock:

  cpu A                                   cpu B
    ipath_mcast_add():
      spin_lock_irq(&mcast_lock);

                                            ipath_mcast_detach():
                                              spin_lock(&n_mcast_grps_lock);

                                            <enter interrupt>

                                            ipath_mcast_find():
                                              spin_lock_irqsave(&mcast_lock);

      spin_lock(&n_mcast_grps_lock);

Fix this by using spin_lock_irq() to take n_mcast_grps_lock.

Signed-off-by: Roland Dreier <rolandd@cisco.com>
„s~@™