NBD: allow nbd to be used locally

This patch allows Network Block Device to be mounted locally (nbd-client to
nbd-server over 127.0.0.1).

It creates a kthread to avoid the deadlock described in NBD tools
documentation.  So, if nbd-client hangs waiting for pages, the kblockd thread
can continue its work and free pages.

I have tested the patch to verify that it avoids the hang that always occurs
when writing to a localhost nbd connection.  I have also tested to verify that
no performance degradation results from the additional thread and queue.

Patch originally from Laurent Vivier.

Signed-off-by: Paul Clements <paul.clements@steeleye.com>
Signed-off-by: Laurent Vivier <Laurent.Vivier@bull.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/include/linux/nbd.h b/include/linux/nbd.h
index 9865720..6907551 100644
--- a/include/linux/nbd.h
+++ b/include/linux/nbd.h
@@ -56,9 +56,11 @@
 	int magic;
 
 	spinlock_t queue_lock;
-	struct list_head queue_head;/* Requests are added here...	*/
+	struct list_head queue_head;	/* Requests waiting result */
 	struct request *active_req;
 	wait_queue_head_t active_wq;
+	struct list_head waiting_queue;	/* Requests to be sent */
+	wait_queue_head_t waiting_wq;
 
 	struct mutex tx_lock;
 	struct gendisk *disk;