f2fs: avoid out-of-range memory access

blkoff_off might over 512 due to fs corrupt or security
vulnerability. That should be checked before being using.

Use ENTRIES_IN_SUM to protect invalid value in cur_data_blkoff.

Signed-off-by: Ocean Chen <oceanchen@google.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>