block: move stats from disk to part0

Move stats related fields - stamp, in_flight, dkstats - from disk to
part0 and unify stat handling such that...

* part_stat_*() now updates part0 together if the specified partition
  is not part0.  ie. part_stat_*() are now essentially all_stat_*().

* {disk|all}_stat_*() are gone.

* part_round_stats() is updated similary.  It handles part0 stats
  automatically and disk_round_stats() is killed.

* part_{inc|dec}_in_fligh() is implemented which automatically updates
  part0 stats for parts other than part0.

* disk_map_sector_rcu() is updated to return part0 if no part matches.
  Combined with the above changes, this makes NULL special case
  handling in callers unnecessary.

* Separate stats show code paths for disk are collapsed into part
  stats show code paths.

* Rename disk_stat_lock/unlock() to part_stat_lock/unlock()

While at it, reposition stat handling macros a bit and add missing
parentheses around macro parameters.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

drivers/md/md.c

@@ -5546,8 +5546,8 @@ static int is_mddev_idle(mddev_t *mddev)
 	rcu_read_lock();
 	rdev_for_each_rcu(rdev, mddev) {
 		struct gendisk *disk = rdev->bdev->bd_contains->bd_disk;
-		curr_events = disk_stat_read(disk, sectors[0]) + 
-				disk_stat_read(disk, sectors[1]) - 
+		curr_events = part_stat_read(&disk->part0, sectors[0]) +
+				part_stat_read(&disk->part0, sectors[1]) -
 				atomic_read(&disk->sync_io);
 		/* sync IO will cause sync_io to increase before the disk_stats
 		 * as sync_io is counted when a request starts, and