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/extents.c b/fs/ext4/extents.c
index 4bb6920..3c0bae1 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -195,8 +195,7 @@
 		if (S_ISREG(inode->i_mode))
 			block_group++;
 	}
-	bg_start = (block_group * EXT4_BLOCKS_PER_GROUP(inode->i_sb)) +
-		le32_to_cpu(EXT4_SB(inode->i_sb)->s_es->s_first_data_block);
+	bg_start = ext4_group_first_block_no(inode->i_sb, block_group);
 	last_block = ext4_blocks_count(EXT4_SB(inode->i_sb)->s_es) - 1;
 
 	/*