block: Update a comment of disk statistics

>From commit 074a7ac ("block: move stats from disk to part0"),
we know that:

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

* disk_stat_lock/unlock() are renamed to part_stat_lock/unlock().

Therefore, outdated comments should be updated accordingly.

Fixes: 074a7ac ("block: move stats from disk to part0")
Signed-off-by: Tang Yizhou <yizhou.tang@shopee.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Tang Yizhou

include/linux/part_stat.h

@@ -17,8 +17,8 @@ struct disk_stats {
 /*
  * Macros to operate on percpu disk statistics:
  *
- * {disk|part|all}_stat_{add|sub|inc|dec}() modify the stat counters and should
- * be called between disk_stat_lock() and disk_stat_unlock().
+ * part_stat_{add|sub|inc|dec}() modify the stat counters and should
+ * be called between part_stat_lock() and part_stat_unlock().
  *
  * part_stat_read() can be called at any time.
  */