loop: Refuse to tear down loop device if partitions are open

loop_clr_fd defers teardown if the loop device itself is
referenced but proceeds when a partition of the loop device is
open. When this happens partition scanning will fail, leaving
behind the block devices for the loop device's partitions.
Prevent this by returning -EBUSY when attempting to tear down
the loop device while any partition block device is open.
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>

drivers/block/loop.c

@@ -843,6 +843,10 @@ static int loop_clr_fd(struct loop_device *lo)
 	if (lo->lo_state != Lo_bound)
 		return -ENXIO;
 
+	/* Fail if any partitions open */
+	if (bdev && bdev->bd_part_count > 0)
+		return -EBUSY;
+
 	/*
 	 * If we've explicitly asked to tear down the loop device,
 	 * and it has an elevated reference count, set it for auto-teardown when