libdiff: Simplify truncation detection

* 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

libdiff: Simplify truncation detection

* 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.

* Adjust the unit test accordingly.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: thj, kevans
Differential Revision: https://reviews.freebsd.org/D55108

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