Fix 32-bit overflow problems: dgrp_t * s_blocks_per_group

Fix 32-bit overflow problems: dgrp_t * s_blocks_per_group

There are a number of places where we multiply a dgrp_t with
s_blocks_per_group expecting that we will get a blk64_t.  This
requires a cast, or using the convenience function
ext2fs_group_first_block2().

This audit was suggested by Eric Sandeen.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>