vm_radix: define vm_radix_insert_lookup_lt and use in vm_page_rename

Use the new pctrie combined lookup/insert. This is an easy application
of the new facility. There are other places where we do this for pages
that may need more plumbing to use combined lookup/insert.

Reviewed by: kib (previous version), dougm, markj
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D45396