LinuxKPI: cleanup internal calls to VM_PAGE_TO_PHYS

Replace FreeBSD native VM_PAGE_TO_PHYS() calls with page_to_phys()
allowing us to work on a struct page in the future using the one
single public Linux KPI interface to map to a native FreeBSD vm_page.

This should be a NOP.

Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D41256