librtld_db: Fix shlib mapping offsets.

kve_offset gives the offset into the backing file, which is not what we
want since different segments may map the same page. Use the base of
the mapping to determine the offset exported by librtld_db instead.

PR: 244732
Reported by: Jenkins, Nicolò Mazzucato <nicomazz97@gmail.com>
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation