mm/backing-dev.c: remove recently-added WARN_ON()

On second thoughts, this is just going to disturb people while telling us
things which we already knew.

Cc: Peter Korsgaard <jacmet@sunsite.dk>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/backing-dev.c

@@ -176,7 +176,7 @@ int bdi_register(struct backing_dev_info *bdi, struct device *parent,
 	int ret = 0;
 	struct device *dev;
 
-	if (WARN_ON(bdi->dev))
+	if (bdi->dev)	/* The driver needs to use separate queues per device */
 		goto exit;
 
 	va_start(args, fmt);