nvmet: move async event work off nvmet-wq

nvmet: move async event work off nvmet-wq

For target nvmet_ctrl_free() flushes ctrl->async_event_work.
If nvmet_ctrl_free() runs on nvmet-wq, the flush re-enters workqueue
completion for the same worker:-

A. Async event work queued on nvmet-wq (prior to disconnect):
  nvmet_execute_async_event()
     queue_work(nvmet_wq, &ctrl->async_event_work)

  nvmet_add_async_event()
     queue_work(nvmet_wq, &ctrl->async_event_work)

B. Full pre-work chain (RDMA CM path):
  nvmet_rdma_cm_handler()
     nvmet_rdma_queue_disconnect()
       __nvmet_rdma_queue_disconnect()
         queue_work(nvmet_wq, &queue->release_work)
           process_one_work()
             lock((wq_completion)nvmet-wq)  <--------- 1st
             nvmet_rdma_release_queue_work()

C. Recursive path (same worker):
  nvmet_rdma_release_queue_work()
     nvmet_rdma_free_queue()
       nvmet_sq_destroy()
         nvmet_ctrl_put()
           nvmet_ctrl_free()
             flush_work(&ctrl->async_event_work)
               __flush_work()
                 touch_wq_lockdep_map()
                 lock((wq_completion)nvmet-wq) <--------- 2nd

Lockdep splat:

  ============================================
  WARNING: possible recursive locking detected
  6.19.0-rc3nvme+ #14 Tainted: G                 N
  --------------------------------------------
  kworker/u192:42/44933 is trying to acquire lock:
  ffff888118a00948 ((wq_completion)nvmet-wq){+.+.}-{0:0}, at: touch_wq_lockdep_map+0x26/0x90

  but task is already holding lock:
  ffff888118a00948 ((wq_completion)nvmet-wq){+.+.}-{0:0}, at: process_one_work+0x53e/0x660

  3 locks held by kworker/u192:42/44933:
   #0: ffff888118a00948 ((wq_completion)nvmet-wq){+.+.}-{0:0}, at: process_one_work+0x53e/0x660
   #1: ffffc9000e6cbe28 ((work_completion)(&queue->release_work)){+.+.}-{0:0}, at: process_one_work+0x1c5/0x660
   #2: ffffffff82d4db60 (rcu_read_lock){....}-{1:3}, at: __flush_work+0x62/0x530

  Workqueue: nvmet-wq nvmet_rdma_release_queue_work [nvmet_rdma]
  Call Trace:
   __flush_work+0x268/0x530
   nvmet_ctrl_free+0x140/0x310 [nvmet]
   nvmet_cq_put+0x74/0x90 [nvmet]
   nvmet_rdma_free_queue+0x23/0xe0 [nvmet_rdma]
   nvmet_rdma_release_queue_work+0x19/0x50 [nvmet_rdma]
   process_one_work+0x206/0x660
   worker_thread+0x184/0x320
   kthread+0x10c/0x240
   ret_from_fork+0x319/0x390

Move async event work to a dedicated nvmet-aen-wq to avoid reentrant
flush on nvmet-wq.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>