cfq: don't register propio policy if !CONFIG_CFQ_GROUP_IOSCHED

cfq has been registering zeroed blkio_poilcy_cfq if CFQ_GROUP_IOSCHED
is disabled.  This fortunately doesn't collide with blk-throtl as
BLKIO_POLICY_PROP is zero but is unnecessary and risky.  Just don't
register it if not enabled.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/cfq-iosched.c

@@ -3878,8 +3878,6 @@ static struct blkio_policy_type blkio_policy_cfq = {
 	},
 	.plid = BLKIO_POLICY_PROP,
 };
-#else
-static struct blkio_policy_type blkio_policy_cfq;
 #endif
 
 static int __init cfq_init(void)
@@ -3910,14 +3908,17 @@ static int __init cfq_init(void)
 		return ret;
 	}
 
+#ifdef CONFIG_CFQ_GROUP_IOSCHED
 	blkio_policy_register(&blkio_policy_cfq);
-
+#endif
 	return 0;
 }
 
 static void __exit cfq_exit(void)
 {
+#ifdef CONFIG_CFQ_GROUP_IOSCHED
 	blkio_policy_unregister(&blkio_policy_cfq);
+#endif
 	elv_unregister(&iosched_cfq);
 	kmem_cache_destroy(cfq_pool);
 }