f2fs-tools: allow unfixed f2fs_checkpoint.checksum_offset

f2fs-tools: allow unfixed f2fs_checkpoint.checksum_offset

Previously, f2fs_checkpoint.checksum_offset points fixed position of
f2fs_checkpoint structure:

"#define CP_CHKSUM_OFFSET	4092"

It is unnecessary, and it breaks the consecutiveness of nat and sit
bitmap stored across checkpoint park block and payload blocks.

This patch allows f2fs-tools to handle unfixed .checksum_offset.

In addition, for the case checksum value is stored in the middle of
checkpoint park, calculating checksum value with superposition method
like we did for inode_checksum.

In addition, add below change:
- using MAX_BITMAP_SIZE_IN_CKPT to clean up codes.
- introduce verify_checksum_chksum() to verify chksum_{offset,value}

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