linuxkpi: Fix set_memory_*

set_memory_* is currently implemented using PHYS_TO_DMAP but not all
architectures have a DMAP. Looking at how this function is used the
given address isn't physical but virtual so the PHYS_TO_DMAP call can
simply be removed.

Also cast numpages before shifting it to avoid overflow.

Reviewed by: kib, markj
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D45057

linuxkpi: Fix uses of `pmap_change_attr()`

[Why]
This function takes an offset and a length as argument, not a physical
address and a number of pages.

This misuse caused the `set_memory_*()` and
`arch_io_reserve_memtype_wc()` functions to return EINVAL.

Another problem was the fact that they returned errors as a positive
integer, whereas Linux uses negative integers.

[How]
Physical addresses and number of pages are converted to offset+length in
the `set_memory_*()` functions.

`arch_io_reserve_memtype_wc()` now calls `pmap_change_attr()` directly
instead of using `set_memory_wc()`.

Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D42053

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

linuxkpi: Add asm/set_memory.h

Provide functions needed for drm-kmod.

MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D35571