block: clarify badblocks lifetime

The badblocks list attached to a gendisk is allocated by the driver
which equates to the driver owning the lifetime of the object.  Do not
automatically free it in del_gendisk(). This is in preparation for
expanding the use of badblocks in libnvdimm drivers and introducing
devm_init_badblocks().
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

block/badblocks.c

@@ -555,6 +555,8 @@ EXPORT_SYMBOL_GPL(badblocks_init);
  */
 void badblocks_exit(struct badblocks *bb)
 {
+	if (!bb)
+		return;
 	kfree(bb->page);
 	bb->page = NULL;
 }

block/genhd.c

@@ -670,11 +670,6 @@ void del_gendisk(struct gendisk *disk)
 	blk_unregister_queue(disk);
 	blk_unregister_region(disk_devt(disk), disk->minors);
 
-	if (disk->bb) {
-		badblocks_exit(disk->bb);
-		kfree(disk->bb);
-	}
-
 	part_stat_set_all(&disk->part0, 0);
 	disk->part0.stamp = 0;