LinuxKPI: Move page_address definition from linux/gfp.h to linux/mm.h

To match Linux. Some future changes may depend on proper location.

Sponsored by: Serenity Cyber Security, LLC
MFC after: 1 week
Reviewed by: bz, emaste
Differential Revision: https://reviews.freebsd.org/D45448

(cherry picked from commit cb8bfc4db8ca8f4ed03406a554e77a013696c8e0)