Btrfs: update the comment of total_bytes and disk_total_bytes of btrfs_devie

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/volumes.h

@@ -64,10 +64,10 @@ struct btrfs_device {
 	/* the internal btrfs device id */
 	u64 devid;
 
-	/* size of the device */
+	/* size of the device in memory */
 	u64 total_bytes;
 
-	/* size of the disk */
+	/* size of the device on disk */
 	u64 disk_total_bytes;
 
 	/* bytes used */