f2fs-tools: fix stat update in sload/fsck

f2fs-tools: fix stat update in sload/fsck

Change logic as below:
- fix to account block/node/inode stats correctly in reserve_new_block()
- check overflow in reserve_new_block()
- move stat update from f2fs_alloc_nid() to reserve_new_block()
- adjust write_checkpoint() to update stat for sload/fsck

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>