erofs/zmap.c: Fix incorrect offset calculation

Vulnerability Description

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

References

• https://git.kernel.org/stable/c/2144859229c1e74f52d3ea067338d314a83a8afb
• https://git.kernel.org/stable/c/9f31d8c889d9a4e47bfcc6c4537d0c9f89fe582c
• https://git.kernel.org/stable/c/6acd87d50998ef0afafc441613aeaf5a8f5c9eff