blkcg: drop BLKCG_STAT_{PRIV|POL|OFF} macros

Now that all stat handling code lives in policy implementations,
there's no need to encode policy ID in cft->private.

* Export blkcg_prfill_[rw]stat() from blkcg, remove
  blkcg_print_[rw]stat(), and implement cfqg_print_[rw]stat() which
  use hard-code BLKIO_POLICY_PROP.

* Use cft->private for offset of the target field directly and drop
  BLKCG_STAT_{PRIV|POL|OFF}().

Signed-off-by: Tejun Heo <tj@kernel.org>
diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index bd6dbfe..6024014 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -941,15 +941,13 @@
 	return __blkg_prfill_rwstat(sf, pdata, &rwstat);
 }
 
-/* print per-cpu blkg_rwstat specified by BLKCG_STAT_PRIV() */
 static int tg_print_cpu_rwstat(struct cgroup *cgrp, struct cftype *cft,
 			       struct seq_file *sf)
 {
 	struct blkio_cgroup *blkcg = cgroup_to_blkio_cgroup(cgrp);
 
-	blkcg_print_blkgs(sf, blkcg, tg_prfill_cpu_rwstat,
-			  BLKCG_STAT_POL(cft->private),
-			  BLKCG_STAT_OFF(cft->private), true);
+	blkcg_print_blkgs(sf, blkcg, tg_prfill_cpu_rwstat, BLKIO_POLICY_THROTL,
+			  cft->private, true);
 	return 0;
 }
 
@@ -1067,14 +1065,12 @@
 	},
 	{
 		.name = "throttle.io_service_bytes",
-		.private = BLKCG_STAT_PRIV(BLKIO_POLICY_THROTL,
-				offsetof(struct tg_stats_cpu, service_bytes)),
+		.private = offsetof(struct tg_stats_cpu, service_bytes),
 		.read_seq_string = tg_print_cpu_rwstat,
 	},
 	{
 		.name = "throttle.io_serviced",
-		.private = BLKCG_STAT_PRIV(BLKIO_POLICY_THROTL,
-				offsetof(struct tg_stats_cpu, serviced)),
+		.private = offsetof(struct tg_stats_cpu, serviced),
 		.read_seq_string = tg_print_cpu_rwstat,
 	},
 	{ }	/* terminate */