nbd: fix possible page fault for nbd disk

When the NBD_CFLAG_DESTROY_ON_DISCONNECT flag is set and at the same
time when the socket is closed due to the server daemon is restarted,
just before the last DISCONNET is totally done if we start a new connection
by using the old nbd_index, there will be crashing randomly, like:

<3>[  110.151949] block nbd1: Receive control failed (result -32)
<1>[  110.152024] BUG: unable to handle page fault for address: 0000058000000840
<1>[  110.152063] #PF: supervisor read access in kernel mode
<1>[  110.152083] #PF: error_code(0x0000) - not-present page
<6>[  110.152094] PGD 0 P4D 0
<4>[  110.152106] Oops: 0000 [#1] SMP PTI
<4>[  110.152120] CPU: 0 PID: 6698 Comm: kworker/u5:1 Kdump: loaded Not tainted 5.3.0-rc4+ #2
<4>[  110.152136] Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
<4>[  110.152166] Workqueue: knbd-recv recv_work [nbd]
<4>[  110.152187] RIP: 0010:__dev_printk+0xd/0x67
<4>[  110.152206] Code: 10 e8 c5 fd ff ff 48 8b 4c 24 18 65 48 33 0c 25 28 00 [...]
<4>[  110.152244] RSP: 0018:ffffa41581f13d18 EFLAGS: 00010206
<4>[  110.152256] RAX: ffffa41581f13d30 RBX: ffff96dd7374e900 RCX: 0000000000000000
<4>[  110.152271] RDX: ffffa41581f13d20 RSI: 00000580000007f0 RDI: ffffffff970ec24f
<4>[  110.152285] RBP: ffffa41581f13d80 R08: ffff96dd7fc17908 R09: 0000000000002e56
<4>[  110.152299] R10: ffffffff970ec24f R11: 0000000000000003 R12: ffff96dd7374e900
<4>[  110.152313] R13: 0000000000000000 R14: ffff96dd7374e9d8 R15: ffff96dd6e3b02c8
<4>[  110.152329] FS:  0000000000000000(0000) GS:ffff96dd7fc00000(0000) knlGS:0000000000000000
<4>[  110.152362] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
<4>[  110.152383] CR2: 0000058000000840 CR3: 0000000067cc6002 CR4: 00000000001606f0
<4>[  110.152401] Call Trace:
<4>[  110.152422]  _dev_err+0x6c/0x83
<4>[  110.152435]  nbd_read_stat.cold+0xda/0x578 [nbd]
<4>[  110.152448]  ? __switch_to_asm+0x34/0x70
<4>[  110.152468]  ? __switch_to_asm+0x40/0x70
<4>[  110.152478]  ? __switch_to_asm+0x34/0x70
<4>[  110.152491]  ? __switch_to_asm+0x40/0x70
<4>[  110.152501]  ? __switch_to_asm+0x34/0x70
<4>[  110.152511]  ? __switch_to_asm+0x40/0x70
<4>[  110.152522]  ? __switch_to_asm+0x34/0x70
<4>[  110.152533]  recv_work+0x35/0x9e [nbd]
<4>[  110.152547]  process_one_work+0x19d/0x340
<4>[  110.152558]  worker_thread+0x50/0x3b0
<4>[  110.152568]  kthread+0xfb/0x130
<4>[  110.152577]  ? process_one_work+0x340/0x340
<4>[  110.152609]  ? kthread_park+0x80/0x80
<4>[  110.152637]  ret_from_fork+0x35/0x40

This is very easy to reproduce by running the nbd-runner.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>