LinuxKPI: replace vtophys+PHYS_TO_VM_PAGE with virt_to_page

Rather than using the FreeBSD internal calls vtophys() and
PHYS_TO_VM_PAGE() use the already existing LinuxKPI abstraction for
this called virt_to_page(). This reduces the amount of compat code
to maintain and will allow further work on struct [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/D41257