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/bitmap.c b/drivers/md/bitmap.c
index 7c426d0..1b1ef31 100644
--- a/drivers/md/bitmap.c
+++ b/drivers/md/bitmap.c
@@ -1207,8 +1207,7 @@
 			prepare_to_wait(&bitmap->overflow_wait, &__wait,
 					TASK_UNINTERRUPTIBLE);
 			spin_unlock_irq(&bitmap->lock);
-			bitmap->mddev->queue
-				->unplug_fn(bitmap->mddev->queue);
+			blk_unplug(bitmap->mddev->queue);
 			schedule();
 			finish_wait(&bitmap->overflow_wait, &__wait);
 			continue;