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 <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Kay Sievers <[e-mail]>
Cc: David Woodhouse <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/backing-dev.c

···
 	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);