libext2fs: use ext2fs_group_first_block() instead of open-coded equivalent

Use ext2fs_group_first_block() instead of the open-coded equivalent in
ext2fs_super_and_bgd_loc() and ext2fs_descriptor_block_loc().

Signed-off-by: Valerie Clement <valerie.clement@bull.net>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
diff --git a/lib/ext2fs/openfs.c b/lib/ext2fs/openfs.c
index 35bd44b..2cc0336 100644
--- a/lib/ext2fs/openfs.c
+++ b/lib/ext2fs/openfs.c
@@ -42,8 +42,7 @@
 	bg = (fs->blocksize / sizeof (struct ext2_group_desc)) * i;
 	if (ext2fs_bg_has_super(fs, bg))
 		has_super = 1;
-	ret_blk = (fs->super->s_first_data_block + has_super + 
-		   (bg * fs->super->s_blocks_per_group));
+	ret_blk = ext2fs_group_first_block(fs, bg) + has_super;
 	/*
 	 * If group_block is not the normal value, we're trying to use
 	 * the backup group descriptors and superblock --- so use the