LU-15002 mke2fs: set free blocks accurately for groups has GDT

LU-15002 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.

Change-Id: Ic26de963ed234af1099d00be6f7b6d0edc057c97
Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-on: https://review.whamcloud.com/c/tools/e2fsprogs/+/52273
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>