mke2fs: set free blocks accurately for groups has GDT

mke2fs: set free blocks accurately for groups has GDT

This patch is part of the preparation required to allow
GDT blocks expand beyond a single group,
it introduces 2 new interfaces:
- ext2fs_count_used_blocks(), to return the blocks used
in the bitmap range.
- ext2fs_reserve_super_and_bgd2() to return blocks used by
superblock/GDT blocks for every group, by looking up blocks used.

Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Link: https://lore.kernel.org/r/20230925060801.1397581-1-dongyangli@ddn.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>