fips: Clarify what to be hashed for the integrity check.

* src/fips.c (get_file_offset): Compute the maximum offset
of segments.
* src/gen-note-integrity.sh: Likewise.

The result is same (in current format of ELF program).
Semantics is more clear. It hashes:

• From the start of shared library file,
• fixed up the ELF header to exclude link-time information,
• up to the last segment.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>