zram: use generic start/end io accounting

Use bio generic_start_io_acct() and generic_end_io_acct() to account
device's block layer statistics.  This will let users to monitor zram
activities using sysstat and similar packages/tools.

Apart from the usual per-stat sysfs attr, zram IO stats are now also
available in '/sys/block/zram<id>/stat' and '/proc/diskstats' files.

We will slowly get rid of per-stat sysfs files.
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/block/zram/zram_drv.c

@@ -670,8 +670,12 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
 static int zram_bvec_rw(struct zram *zram, struct bio_vec *bvec, u32 index,
 			int offset, int rw)
 {
+	unsigned long start_time = jiffies;
 	int ret;
 
+	generic_start_io_acct(rw, bvec->bv_len >> SECTOR_SHIFT,
+			&zram->disk->part0);
+
 	if (rw == READ) {
 		atomic64_inc(&zram->stats.num_reads);
 		ret = zram_bvec_read(zram, bvec, index, offset);
@@ -680,6 +684,8 @@ static int zram_bvec_rw(struct zram *zram, struct bio_vec *bvec, u32 index,
 		ret = zram_bvec_write(zram, bvec, index, offset);
 	}
 
+	generic_end_io_acct(rw, &zram->disk->part0, start_time);
+
 	if (unlikely(ret)) {
 		if (rw == READ)
 			atomic64_inc(&zram->stats.failed_reads);