[BLOCK] Document the READ/WRITE splitup of the disk stats

Use the symbolic name where appropriate and add a comment to the
disk_stats structure.
Signed-off-by: Jens Axboe <axboe@suse.de>

block/genhd.c

@@ -391,12 +391,14 @@ static ssize_t disk_stats_read(struct gendisk * disk, char *page)
 		"%8u %8u %8llu %8u "
 		"%8u %8u %8u"
 		"\n",
-		disk_stat_read(disk, ios[0]), disk_stat_read(disk, merges[0]),
-		(unsigned long long)disk_stat_read(disk, sectors[0]),
-		jiffies_to_msecs(disk_stat_read(disk, ticks[0])),
-		disk_stat_read(disk, ios[1]), disk_stat_read(disk, merges[1]),
-		(unsigned long long)disk_stat_read(disk, sectors[1]),
-		jiffies_to_msecs(disk_stat_read(disk, ticks[1])),
+		disk_stat_read(disk, ios[READ]),
+		disk_stat_read(disk, merges[READ]),
+		(unsigned long long)disk_stat_read(disk, sectors[READ]),
+		jiffies_to_msecs(disk_stat_read(disk, ticks[READ])),
+		disk_stat_read(disk, ios[WRITE]),
+		disk_stat_read(disk, merges[WRITE]),
+		(unsigned long long)disk_stat_read(disk, sectors[WRITE]),
+		jiffies_to_msecs(disk_stat_read(disk, ticks[WRITE])),
 		disk->in_flight,
 		jiffies_to_msecs(disk_stat_read(disk, io_ticks)),
 		jiffies_to_msecs(disk_stat_read(disk, time_in_queue)));

include/linux/genhd.h

@@ -78,7 +78,7 @@ struct hd_struct {
 	sector_t start_sect;
 	sector_t nr_sects;
 	struct kobject kobj;
-	unsigned ios[2], sectors[2];
+	unsigned ios[2], sectors[2];	/* READs and WRITEs */
 	int policy, partno;
 };
 
@@ -89,7 +89,7 @@ struct hd_struct {
 #define GENHD_FL_SUPPRESS_PARTITION_INFO	32
 
 struct disk_stats {
-	unsigned sectors[2];
+	unsigned sectors[2];		/* READs and WRITEs */
 	unsigned ios[2];
 	unsigned merges[2];
 	unsigned ticks[2];