[v21.11.x] storage: fix integer overflow in max_timestamp

[jcsp]

Cover letter

Backport of PR #4010
Fixes #4112

(cherry picked from commit 3aacf0d)

This fixes:

• Customer sees "segment with bogus max timestamp" warnings with partitions written via MirrorMaker 2
• retention.ms cleanup not working on such segments.

Release notes

Improvements

• An issue is fixed where out-of-order timestamps in input data could cause time-based retention cleanup to fail, and warning messages related to "bogus max timestamp".