libext2fs: Add EXT2_DESC_SIZE and EXT2_DESC_PER_BLOCK macros

Add macros to support variable-length group descriptors for ext4.

Signed-off-by: Valerie Clement <valerie.clement@bull.net>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
diff --git a/lib/ext2fs/initialize.c b/lib/ext2fs/initialize.c
index 16e9eaa..a7f777b 100644
--- a/lib/ext2fs/initialize.c
+++ b/lib/ext2fs/initialize.c
@@ -67,7 +67,7 @@
 {
 	struct ext2_super_block *sb = fs->super;
 	unsigned long bpg = sb->s_blocks_per_group;
-	unsigned int gdpb = fs->blocksize / sizeof(struct ext2_group_desc);
+	unsigned int gdpb = EXT2_DESC_PER_BLOCK(sb);
 	unsigned long max_blocks = 0xffffffff;
 	unsigned long rsv_groups;
 	unsigned int rsv_gdb;