LinuxKPI: add virt_to_phys()

Add virt_to_phys() as a define to vtophys().
This is used by a wireless driver for dma related work; sigh.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D37301

(cherry picked from commit b5c3819c7b57d7e3aa94f1db779648bb9723c1f5)