LinuxKPI: reduce usage of struct vm_page and vm_page_t

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

(cherry picked from commit 9e9c682ff3a1aec20a7e5f5b1dd88ceaec7c3950)