imgact_elf: Check note body sizes

In parse_notes we validate that the note name fits within the note
buffer, but we do not do the same for the note data, so there is some
potential for an OOB read in the note handler. Add a bounds check.

Reported by: Ilja Van Sprundel <ivansprundel@ioactive.com>
Reviewed by: kib, emaste
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D53063