Add UNPLUG traces to all appropriate places

Added blk_unplug interface, allowing all invocations of unplugs to result
in a generated blktrace UNPLUG.

Signed-off-by: Alan D. Brunelle <Alan.Brunelle@hp.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index fc6607a..5cdcc93 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c
@@ -593,8 +593,7 @@
 			atomic_inc(&rdev->nr_pending);
 			rcu_read_unlock();
 
-			if (r_queue->unplug_fn)
-				r_queue->unplug_fn(r_queue);
+			blk_unplug(r_queue);
 
 			rdev_dec_pending(rdev, mddev);
 			rcu_read_lock();