kldxref: Fix maketempfile function's way of finding the root dir

Rather than assuming that the "root" is passed as directory and will be
marked by a trailing slash, we just assume that the directory, which has
been checked previously to be a directory, is a directory.

This fixes an inconsistency between kldxref /boot/modules, which tries
to create the temp file in /boot/, and kldxref /boot/modules/, which
tries to create it in /boot/modules/ itself.

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1093