blkcg: fix minor bug in blkg_alloc()

blkg_alloc() was mistakenly checking blkcg_policy_enabled() twice.
The latter test should have been on whether pol->pd_init_fn() exists.
This doesn't cause actual problems because both blkcg policies
implement pol->pd_init_fn().  Fix it.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Vivek Goyal <vgoyal@redhat.com>

block/blk-cgroup.c

@@ -114,7 +114,7 @@ static struct blkcg_gq *blkg_alloc(struct blkcg *blkcg, struct request_queue *q,
 		pd->blkg = blkg;
 
 		/* invoke per-policy init */
-		if (blkcg_policy_enabled(blkg->q, pol))
+		if (pol->pd_init_fn)
 			pol->pd_init_fn(blkg);
 	}