nbd: call genl_unregister_family() first in nbd_cleanup()

Otherwise there may be race between module removal and the handling of
netlink command, which can lead to the oops as shown below:

  BUG: kernel NULL pointer dereference, address: 0000000000000098
  Oops: 0002 [#1] SMP PTI
  CPU: 1 PID: 31299 Comm: nbd-client Tainted: G            E     5.14.0-rc4
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996)
  RIP: 0010:down_write+0x1a/0x50
  Call Trace:
   start_creating+0x89/0x130
   debugfs_create_dir+0x1b/0x130
   nbd_start_device+0x13d/0x390 [nbd]
   nbd_genl_connect+0x42f/0x748 [nbd]
   genl_family_rcv_msg_doit.isra.0+0xec/0x150
   genl_rcv_msg+0xe5/0x1e0
   netlink_rcv_skb+0x55/0x100
   genl_rcv+0x29/0x40
   netlink_unicast+0x1a8/0x250
   netlink_sendmsg+0x21b/0x430
   ____sys_sendmsg+0x2a4/0x2d0
   ___sys_sendmsg+0x81/0xc0
   __sys_sendmsg+0x62/0xb0
   __x64_sys_sendmsg+0x1f/0x30
   do_syscall_64+0x3b/0xc0
   entry_SYSCALL_64_after_hwframe+0x44/0xae
  Modules linked in: nbd(E-)
Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/20220521073749.3146892-2-yukuai3@huawei.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/nbd.c

@@ -2528,6 +2528,12 @@ static void __exit nbd_cleanup(void)
 	struct nbd_device *nbd;
 	LIST_HEAD(del_list);
 
+	/*
+	 * Unregister netlink interface prior to waiting
+	 * for the completion of netlink commands.
+	 */
+	genl_unregister_family(&nbd_genl_family);
+
 	nbd_dbg_close();
 
 	mutex_lock(&nbd_index_mutex);
@@ -2546,7 +2552,6 @@ static void __exit nbd_cleanup(void)
 	destroy_workqueue(nbd_del_wq);
 
 	idr_destroy(&nbd_index_idr);
-	genl_unregister_family(&nbd_genl_family);
 	unregister_blkdev(NBD_MAJOR, "nbd");
 }