CVE-2022-49747: Undefined Security Weakness

In the Linux kernel, the following vulnerability has been resolved:

erofs/zmap.c: Fix incorrect offset calculation

Effective offset to add to length was being incorrectly calculated, which resulted in iomap->length being set to 0, triggering a WARN_ON in iomap_iter_done().

Fix that, and describe it in comments.

This was reported as a crash by syzbot under an issue about a warning encountered in iomap_iter_done(), but unrelated to erofs.

C reproducer: https://syzkaller.appspot.com/text?tag=ReproC&x=1037a6b2880000 Kernel config: https://syzkaller.appspot.com/text?tag=KernelConfig&x=e2021a61197ebe02 Dashboard link: https://syzkaller.appspot.com/bug?extid=a8e049cd3abd342936b6