ddb show pte: use pmap of kdb_thread

show pte from the pmap of the process of the current DDB thread, instead
of necessarily the PCPU pmap.

Submitted by: Ryan Libby <rlibby@gmail.com>
Reviewed by: kib
MFC after: 1 week
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D9645