md: the bitmap code needs to use blk_plug_device_unlocked()

It doesn't hold the queue lock, so it's both racey on the queue flags
and thus spews a warning.
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

drivers/md/bitmap.c

@@ -1234,7 +1234,7 @@ int bitmap_startwrite(struct bitmap *bitmap, sector_t offset, unsigned long sect
 		case 0:
 			bitmap_file_set_bit(bitmap, offset);
 			bitmap_count_page(bitmap,offset, 1);
-			blk_plug_device(bitmap->mddev->queue);
+			blk_plug_device_unlocked(bitmap->mddev->queue);
 			/* fall through */
 		case 1:
 			*bmc = 2;