[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>
diff --git a/block/genhd.c b/block/genhd.c
index 54aec4a..f04609d 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -391,12 +391,14 @@
 		"%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)));