nbd: do del_gendisk() asynchronously for NBD_DESTROY_ON_DISCONNECT

Now open_mutex is used to synchronize partition operations (e.g,
blk_drop_partitions() and blkdev_reread_part()), however it makes
nbd driver broken, because nbd may call del_gendisk() in nbd_release()
or nbd_genl_disconnect() if NBD_CFLAG_DESTROY_ON_DISCONNECT is enabled,
and deadlock occurs, as shown below:

// AB-BA dead-lock
nbd_genl_disconnect            blkdev_open
  nbd_disconnect_and_put
                                 lock bd_mutex
  // last ref
  nbd_put
    lock nbd_index_mutex
      del_gendisk
                                   nbd_open
                                     try lock nbd_index_mutex
        try lock bd_mutex

 or

// AA dead-lock
nbd_release
  lock bd_mutex
    nbd_put
      try lock bd_mutex

Instead of fixing block layer (e.g, introduce another lock), fixing
the nbd driver to call del_gendisk() in a kworker when
NBD_DESTROY_ON_DISCONNECT is enabled. When NBD_DESTROY_ON_DISCONNECT
is disabled, nbd device will always be destroy through module removal,
and there is no risky of deadlock.

To ensure the reuse of nbd index succeeds, moving the calling of
idr_remove() after del_gendisk(), so if the reused index is not found
in nbd_index_idr, the old disk must have been deleted. And reusing
the existing destroy_complete mechanism to ensure nbd_genl_connect()
will wait for the completion of del_gendisk().

Also adding a new workqueue for nbd removal, so nbd_cleanup()
can ensure all removals complete before exits.

Reported-by:  <syzbot+0fe7752e52337864d29b@syzkaller.appspotmail.com>
Fixes: c76f48eb

 ("block: take bd_mutex around delete_partitions in del_gendisk")
Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210811124428.2368491-2-hch@lst.de
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>