LinuxKPI: cleanup internal calls to VM_PAGE_TO_PHYS

Replace FreeBSD native VM_PAGE_TO_PHYS() calls with page_to_phys()
allowing us to work on a struct page in the future using the one
single public Linux KPI interface to map to a native FreeBSD 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/D41256

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
MFC after: 2 weeks
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D41255

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

linuxkpi: Add `kmap_local_page_prot()` and `kunmap_local()`

They are synonymous to `kmap_atomic_prot()` and `kunmap_atomic()`
respectively.

While here, fix several style(9) issues.

Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D38155

LinuxKPI: Constantly use _LINUXKPI_ prefix in include guards

MFC after: 1 week
Reviewed by: bz, emaste, hselasky, manu
Differential Revision: https://reviews.freebsd.org/D33562

LinuxKPI: Import linux_page.c and some dependent code from drm-kmod

No functional changes intended

Reviewed by: hselasky, manu, markj
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D32167