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>

block/genhd.c

@@ -671,7 +671,7 @@ void del_gendisk(struct gendisk *disk)
 	blk_unregister_region(disk_devt(disk), disk->minors);
 
 	if (disk->bb) {
-		badblocks_free(disk->bb);
+		badblocks_exit(disk->bb);
 		kfree(disk->bb);
 	}