resume: Backward compatibility for resume_offset

resume: Backward compatibility for resume_offset

In Debian we will need to maintain backward compatibility with Linux
4.9 at least until after the "buster" release.  Therefore we need to
accept that /sys/power/resume_offset might not exist.

If we can't open that file because it doesn't exist, we should check
whether the offset we are trying to set is the offset that the kernel
would use anyway.  In that case, continue.

Gbp-Pq: Name resume-backward-compatibility-for-resume_offset.patch