IB/ipoib: Fix mcast list locking

IB/ipoib: Fix mcast list locking

[ Upstream commit 4f973e211b3b1c6d36f7c6a19239d258856749f9 ]

Releasing the `priv->lock` while iterating the `priv->multicast_list` in
`ipoib_mcast_join_task()` opens a window for `ipoib_mcast_dev_flush()` to
remove the items while in the middle of iteration. If the mcast is removed
while the lock was dropped, the for loop spins forever resulting in a hard
lockup (as was reported on RHEL 4.18.0-372.75.1.el8_6 kernel):

    Task A (kworker/u72:2 below)       | Task B (kworker/u72:0 below)
    -----------------------------------+-----------------------------------
    ipoib_mcast_join_task(work)        | ipoib_ib_dev_flush_light(work)
      spin_lock_irq(&priv->lock)       | __ipoib_ib_dev_flush(priv, ...)
      list_for_each_entry(mcast,       | ipoib_mcast_dev_flush(dev = priv->dev)
          &priv->multicast_list, list) |
        ipoib_mcast_join(dev, mcast)   |
          spin_unlock_irq(&priv->lock) |
                                       |   spin_lock_irqsave(&priv->lock, flags)
                                       |   list_for_each_entry_safe(mcast, tmcast,
                                       |                  &priv->multicast_list, list)
                                       |     list_del(&mcast->list);
                                       |     list_add_tail(&mcast->list, &remove_list)
                                       |   spin_unlock_irqrestore(&priv->lock, flags)
          spin_lock_irq(&priv->lock)   |
                                       |   ipoib_mcast_remove_list(&remove_list)
   (Here, `mcast` is no longer on the  |     list_for_each_entry_safe(mcast, tmcast,
    `priv->multicast_list` and we keep |                            remove_list, list)
    spinning on the `remove_list` of   |  >>>  wait_for_completion(&mcast->done)
    the other thread which is blocked  |
    and the list is still valid on     |
    it's stack.)

Fix this by keeping the lock held and changing to GFP_ATOMIC to prevent
eventual sleeps.
Unfortunately we could not reproduce the lockup and confirm this fix but
based on the code review I think this fix should address such lockups.

crash> bc 31
PID: 747      TASK: ff1c6a1a007e8000  CPU: 31   COMMAND: "kworker/u72:2"
--
    [exception RIP: ipoib_mcast_join_task+0x1b1]
    RIP: ffffffffc0944ac1  RSP: ff646f199a8c7e00  RFLAGS: 00000002
    RAX: 0000000000000000  RBX: ff1c6a1a04dc82f8  RCX: 0000000000000000
                                  work (&priv->mcast_task{,.work})
    RDX: ff1c6a192d60ac68  RSI: 0000000000000286  RDI: ff1c6a1a04dc8000
           &mcast->list
    RBP: ff646f199a8c7e90   R8: ff1c699980019420   R9: ff1c6a1920c9a000
    R10: ff646f199a8c7e00  R11: ff1c6a191a7d9800  R12: ff1c6a192d60ac00
                                                         mcast
    R13: ff1c6a1d82200000  R14: ff1c6a1a04dc8000  R15: ff1c6a1a04dc82d8
           dev                    priv (&priv->lock)     &priv->multicast_list (aka head)
    ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018

Signed-off-by: Sasha Levin <sashal@kernel.org>