f2fs: 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 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.

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

include/linux/f2fs_fs.h

@@ -164,6 +164,10 @@ struct f2fs_checkpoint {
 	unsigned char sit_nat_version_bitmap[1];
 } __packed;
 
+#define CP_CHKSUM_OFFSET	4092	/* default chksum offset in checkpoint */
+#define CP_MIN_CHKSUM_OFFSET						\
+	(offsetof(struct f2fs_checkpoint, sit_nat_version_bitmap))
+
 /*
  * For orphan inode management
  */