nbd: do not allow two clients at the same time

Two nbd-clients at same time are bad idea, and cause WARN_ON from nbd in
2.6.28-rc7 from sysfs_add_one.  This simply prevents that from happening.

To reproduce:

 cat /dev/zero | head -c 10000000 > /tmp/delme.fstest.fs
 nbd-server 9100 -l /anyone.can.connect > /tmp/delme.fstest.fs &
 sleep 1
 nbd-client localhost 9100 /dev/nd0 &
 nbd-client localhost 9100 /dev/nd0 &

Signed-off-by: Pavel Machek <pavel@suse.cz>
Acked-by: Paul Clements <paul.clements@steeleye.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/block/nbd.c

@@ -406,13 +406,15 @@ static int nbd_do_it(struct nbd_device *lo)
 	ret = sysfs_create_file(&disk_to_dev(lo->disk)->kobj, &pid_attr.attr);
 	if (ret) {
 		printk(KERN_ERR "nbd: sysfs_create_file failed!");
+		lo->pid = 0;
 		return ret;
 	}
 
 	while ((req = nbd_read_stat(lo)) != NULL)
 		nbd_end_request(req);
 
 	sysfs_remove_file(&disk_to_dev(lo->disk)->kobj, &pid_attr.attr);
+	lo->pid = 0;
 	return 0;
 }
 
@@ -648,6 +650,8 @@ static int nbd_ioctl(struct block_device *bdev, fmode_t mode,
 		set_capacity(lo->disk, lo->bytesize >> 9);
 		return 0;
 	case NBD_DO_IT:
+		if (lo->pid)
+			return -EBUSY;
 		if (!lo->file)
 			return -EINVAL;
 		thread = kthread_create(nbd_thread, lo, lo->disk->disk_name);