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/misc/mke2fs.c b/misc/mke2fs.c
index 4a6cace..be40fcd 100644
--- a/misc/mke2fs.c
+++ b/misc/mke2fs.c
@@ -693,8 +693,7 @@
 	if (s->s_reserved_gdt_blocks)
 		printf(_("Maximum filesystem blocks=%lu\n"),
 		       (s->s_reserved_gdt_blocks + fs->desc_blocks) *
-		       (fs->blocksize / sizeof(struct ext2_group_desc)) *
-		       s->s_blocks_per_group);
+		       EXT2_DESC_PER_BLOCK(s) * s->s_blocks_per_group);
 	if (fs->group_desc_count > 1)
 		printf(_("%u block groups\n"), fs->group_desc_count);
 	else
@@ -826,7 +825,7 @@
 			bpg = param->s_blocks_per_group;
 			if (!bpg)
 				bpg = blocksize * 8;
-			gdpb = blocksize / sizeof(struct ext2_group_desc);
+			gdpb = EXT2_DESC_PER_BLOCK(param);
 			group_desc_count = 
 				ext2fs_div_ceil(param->s_blocks_count, bpg);
 			desc_blocks = (group_desc_count +