dm: remove pointless kobject comparison in dm_get_from_kobject

The comparison is always true and the compiler optimizes it out anyway.

Milan offered additional context relative to the original commit
784aae73 ("dm: add name and uuid to sysfs") which introduced the code:
"I think it is just relict of some experiments before I committed this
simple embedded sysfs kobj handling".
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Acked-by: Milan Broz <gmazyland@gmail.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm.c

@@ -2905,17 +2905,11 @@ struct kobject *dm_kobject(struct mapped_device *md)
 	return &md->kobj;
 }
 
-/*
- * struct mapped_device should not be exported outside of dm.c
- * so use this check to verify that kobj is part of md structure
- */
 struct mapped_device *dm_get_from_kobject(struct kobject *kobj)
 {
 	struct mapped_device *md;
 
 	md = container_of(kobj, struct mapped_device, kobj);
-	if (&md->kobj != kobj)
-		return NULL;
 
 	if (test_bit(DMF_FREEING, &md->flags) ||
 	    dm_deleting_md(md))