dm cache: destroy migration_cache if cache target registration failed

Commit 7e6358d244e47 ("dm: fix various targets to dm_register_target
after module __init resources created") inadvertently introduced this
bug when it moved dm_register_target() after the call to KMEM_CACHE().

Fixes: 7e6358d244e47 ("dm: fix various targets to dm_register_target after module __init resources created")
Cc: stable@vger.kernel.org
Signed-off-by: Shenghui Wang <shhuiw@foxmail.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-cache-target.c

@@ -3484,14 +3484,13 @@ static int __init dm_cache_init(void)
 	int r;
 
 	migration_cache = KMEM_CACHE(dm_cache_migration, 0);
-	if (!migration_cache) {
-		dm_unregister_target(&cache_target);
+	if (!migration_cache)
 		return -ENOMEM;
-	}
 
 	r = dm_register_target(&cache_target);
 	if (r) {
 		DMERR("cache target registration failed: %d", r);
+		kmem_cache_destroy(migration_cache);
 		return r;
 	}