ext2fs_cg_get_csum
gd->ext2bgd_checksum = ext2fs_cg_get_csum(fs, cg, gd);
uint16_t csum = ext2fs_cg_get_csum(fs, cg, gd);
gd->ext2bgd_checksum = ext2fs_cg_get_csum(fs, cg, gd);
static uint16_t ext2fs_cg_get_csum(struct m_ext2fs *, int, struct ext2_gd *);