f2fs: use crc and cp version to determine roll-forward recovery

Previously, we used cp_version only to detect recoverable dnodes.
In order to avoid same garbage cp_version, we needed to truncate the next
dnode during checkpoint, resulting in additional discard or data write.
If we can distinguish this by using crc in addition to cp_version, we can
remove this overhead.

There is backward compatibility concern where it changes node_footer layout.
So, this patch introduces a new checkpoint flag, CP_CRC_RECOVERY_FLAG, to
detect new layout. New layout will be activated only when this flag is set.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

include/linux/f2fs_fs.h

@@ -100,6 +100,7 @@ struct f2fs_super_block {
 /*
  * For checkpoint
  */
+#define CP_CRC_RECOVERY_FLAG	0x00000040
 #define CP_FASTBOOT_FLAG	0x00000020
 #define CP_FSCK_FLAG		0x00000010
 #define CP_ERROR_FLAG		0x00000008