fsck.f2fs: fix invalidate checkpoint

If one cp is invalidate, set CP_FSCK_FLAG to allow fsck_verify() to
fixing cp at the end of check.

Signed-off-by: Sheng Yong <shengyong1@xiaomi.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>