vm_page_replace: add wrapper to KASSERT about old page

It turns out the callers of vm_page_replace know exactly which page they are
replacing and would like to assert about it. Change those from hard panics to
KASSERTs, and provide them with a wrapper so they don't have to deal with
warnings from an INVARIANTS-dependent dead store of the return value of
vm_page_replace.

Submitted by: Ryan Libby <rlibby@gmail.com>
Reviewed by: alc, kib (earlier version)
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D4497