We currently define (Linux) page to (FreeBSD) vm_page.
Cleanup some of the direct struct vm_page and vm_page_t declarations
and usages in the Linux KPI and make them 'struct page' or
'struct page *' to prepare for more upcoming work.
This should be a NOP.
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
(cherry picked from commit 13cea84defda2c586945d6f9df19f57a496a87ce)