nbd: fix I/O hang on disconnected nbds

Fix a problem that causes I/O to a disconnected (or partially initialized)
nbd device to hang indefinitely.  To reproduce:

# ioctl NBD_SET_SIZE_BLOCKS /dev/nbd23 514048
# dd if=/dev/nbd23 of=/dev/null bs=4096 count=1

...hangs...

This can also occur when an nbd device loses its nbd-client/server
connection.  Although we clear the queue of any outstanding I/Os after the
client/server connection fails, any additional I/Os that get queued later
will hang.

This bug may also be the problem reported in this bug report:
http://bugzilla.kernel.org/show_bug.cgi?id=12277

Testing would need to be performed to determine if the two issues are the
same.

This problem was introduced by the new request handling thread code ("NBD:
allow nbd to be used locally", 3/2008), which entered into mainline around
2.6.25.

The fix, which is fairly simple, is to restore the check for lo->sock
being NULL in do_nbd_request.  This causes I/O to an uninitialized nbd to
immediately fail with an I/O error, as it did prior to the introduction of
this bug.

Signed-off-by: Paul Clements <paul.clements@steeleye.com>
Reported-by: Jon Nelson <jnelson-kernel-bugzilla@jamponi.net>
Acked-by: Pavel Machek <pavel@ucw.cz>
Cc: <stable@kernel.org>		[2.6.26.x, 2.6.27.x, 2.6.28.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/block/nbd.c

@@ -549,6 +549,15 @@ static void do_nbd_request(struct request_queue * q)
 
 		BUG_ON(lo->magic != LO_MAGIC);
 
+		if (unlikely(!lo->sock)) {
+			printk(KERN_ERR "%s: Attempted send on closed socket\n",
+				lo->disk->disk_name);
+			req->errors++;
+			nbd_end_request(req);
+			spin_lock_irq(q->queue_lock);
+			continue;
+		}
+
 		spin_lock_irq(&lo->queue_lock);
 		list_add_tail(&req->queuelist, &lo->waiting_queue);
 		spin_unlock_irq(&lo->queue_lock);