nbd: Fix hung when signal interrupts nbd_start_device_ioctl()

syzbot reported hung task [1].  The following program is a simplified
version of the reproducer:

int main(void)
{
	int sv[2], fd;

	if (socketpair(AF_UNIX, SOCK_STREAM, 0, sv) < 0)
		return 1;
	if ((fd = open("/dev/nbd0", 0)) < 0)
		return 1;
	if (ioctl(fd, NBD_SET_SIZE_BLOCKS, 0x81) < 0)
		return 1;
	if (ioctl(fd, NBD_SET_SOCK, sv[0]) < 0)
		return 1;
	if (ioctl(fd, NBD_DO_IT) < 0)
		return 1;
	return 0;
}

When signal interrupt nbd_start_device_ioctl() waiting the condition
atomic_read(&config->recv_threads) == 0, the task can hung because it
waits the completion of the inflight IOs.

This patch fixes the issue by clearing queue, not just shutdown, when
signal interrupt nbd_start_device_ioctl().

Link: https://syzkaller.appspot.com/bug?id=7d89a3ffacd2b83fdd39549bc4d8e0a89ef21239 [1]
Reported-by: syzbot+38e6c55d4969a14c1534@syzkaller.appspotmail.com
Signed-off-by: Shigeru Yoshida <syoshida@redhat.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/20220907163502.577561-1-syoshida@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Shigeru Yoshida

drivers/block/nbd.c

@@ -1413,10 +1413,12 @@ static int nbd_start_device_ioctl(struct nbd_device *nbd)
 	mutex_unlock(&nbd->config_lock);
 	ret = wait_event_interruptible(config->recv_wq,
 					 atomic_read(&config->recv_threads) == 0);
-	if (ret)
+	if (ret) {
 		sock_shutdown(nbd);
-	flush_workqueue(nbd->recv_workq);
+		nbd_clear_que(nbd);
+	}
 
+	flush_workqueue(nbd->recv_workq);
 	mutex_lock(&nbd->config_lock);
 	nbd_bdev_reset(nbd);
 	/* user requested, ignore socket errors */