Revert "block: fix accounting bug on cross partition merges"

This reverts commit 7681bfeecc.

Conflicts:

	include/linux/genhd.h

It has numerous issues with the cleanup path and non-elevator
devices. Revert it for now so we can come up with a clean
version without rushing things.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

block/blk-merge.c

@@ -351,7 +351,7 @@ static void blk_account_io_merge(struct request *req)
 		int cpu;
 
 		cpu = part_stat_lock();
-		part = req->part;
+		part = disk_map_sector_rcu(req->rq_disk, blk_rq_pos(req));
 
 		part_round_stats(cpu, part);
 		part_dec_in_flight(part, rq_data_dir(req));