block: misc updates

This patch makes the following misc updates in preparation for
disk->part dereference fix and extended block devt support.

* implment part_to_disk()

* fix comment about gendisk->part indexing

* rename get_part() to disk_map_sector()

* don't use n which is always zero while printing disk information in
  diskstats_show()

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/drivers/block/aoe/aoecmd.c b/drivers/block/aoe/aoecmd.c
index 2f17462..885d140 100644
--- a/drivers/block/aoe/aoecmd.c
+++ b/drivers/block/aoe/aoecmd.c
@@ -757,7 +757,7 @@
 	const int rw = bio_data_dir(bio);
 	struct hd_struct *part;
 
-	part = get_part(disk, sector);
+	part = disk_map_sector(disk, sector);
 	all_stat_inc(disk, part, ios[rw], sector);
 	all_stat_add(disk, part, ticks[rw], duration, sector);
 	all_stat_add(disk, part, sectors[rw], n_sect, sector);