libdiff: Simplify truncation detection

[des]

Summary

• Adjust the comment to reflect the fact that SIGBUS can occur not only if the file is truncated, but also if an I/O error occurs while paging in any part of it.

• Instead of setting a flag, just return EIO.

MFC after: 1 week
Sponsored by: Klara, Inc.

[des]

adjust unit test