badblocks: rename badblocks_free to badblocks_exit

For symmetry with badblocks_init() make it clear that this path only
destroys incremental allocations of a badblocks instance, and does not
free the badblocks instance itself.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
diff --git a/drivers/md/md.c b/drivers/md/md.c
index 1e48aa9..96a9918 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -710,7 +710,7 @@
 		put_page(rdev->bb_page);
 		rdev->bb_page = NULL;
 	}
-	badblocks_free(&rdev->badblocks);
+	badblocks_exit(&rdev->badblocks);
 }
 EXPORT_SYMBOL_GPL(md_rdev_clear);