blkcg: make blkcg_activate_policy() allow NULL ->pd_init_fn

blkg_create() allows NULL ->pd_init_fn() but blkcg_activate_policy()
doesn't.  As both in-kernel policies implement ->pd_init_fn, it
currently doesn't break anything.  Update blkcg_activate_policy() so
that its behavior is consistent with blkg_create().

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-cgroup.c

@@ -1083,7 +1083,8 @@ pd_prealloc:
 		blkg->pd[pol->plid] = pd;
 		pd->blkg = blkg;
 		pd->plid = pol->plid;
-		pol->pd_init_fn(blkg);
+		if (pol->pd_init_fn)
+			pol->pd_init_fn(blkg);
 	}
 
 	__set_bit(pol->plid, q->blkcg_pols);