ext2_cg_number_gdb
number += ext2_cg_number_gdb(fs, cg);
number += ext2_cg_number_gdb(fs, cg);
uint64_t ext2_cg_number_gdb(struct m_ext2fs *fs, int cg);
last_cg_block = ext2_cg_number_gdb(fs, 0);