ext4: cleanup to use ext4_group_first_block_no()

This is a cleanup and simplification patch which takes some open-coded
calculations to calculate the first block number of a group and
converts them to use the (already defined) ext4_group_first_block_no()
function.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: Andreas Dilger <adilger@sun.com>
diff --git a/fs/ext4/balloc.c b/fs/ext4/balloc.c
index 720061a..fadbc4d 100644
--- a/fs/ext4/balloc.c
+++ b/fs/ext4/balloc.c
@@ -130,8 +130,7 @@
 		 * to make sure we calculate the right free blocks
 		 */
 		group_blocks = ext4_blocks_count(sbi->s_es) -
-			le32_to_cpu(sbi->s_es->s_first_data_block) -
-			(EXT4_BLOCKS_PER_GROUP(sb) * (ngroups - 1));
+			ext4_group_first_block_no(sb, ngroups - 1);
 	} else {
 		group_blocks = EXT4_BLOCKS_PER_GROUP(sb);
 	}