[PATCH] unplugging fix

Patch from Neil Brown <neilb@cse.unsw.edu.au>

Allow auto-unplugging to work for devices that do it themselves.

Auto-unplugging - and blk_unplug_work in particular - assumes that the device
uses "generic_unplug_device" for unplugging, but some devices don't.  md
crashes.

So blk_unplug_work should use ->unplug_fn

drivers/block/ll_rw_blk.c

@@ -1040,7 +1040,8 @@ void generic_unplug_device(void *data)
 
 static void blk_unplug_work(void *data)
 {
-	generic_unplug_device(data);
+	request_queue_t *q = data;
+	q->unplug_fn(q);
 }
 
 static void blk_unplug_timeout(unsigned long data)