linuxkpi: Move LINUXKPI_PROT_VALID higher

[andrew]

Summary

Move LINUXKPI_PROT_VALID and LINUXKPI_CACHE_MODE_SHIFT out of the
bottom 8 bits. This allows the entire vm_prot_t to be stored in the
LinuxKPI pgprot_t.

This will be needed when we add more flags to vm_prot_t, e.g. for MTE
on arm64.

Sponsored by: Arm Ltd

[andrew]

Is anyone who knows about LinuxKPI able to check this? I don't have anything that uses it, but need to move the value for MTE.

[bz]

This came from 1ea4c85781e94 and was revised in ebf854802d83a.

@ashafer has an open review ( https://reviews.freebsd.org/D59436 ) and I was trying to understand some of this recently.
If you can give me a few days I'll try to fully wrap my head around this.

I assume @ashafer and @dumbbell can test this for nvidia and i915/amdgpu.

[ashafer]

This seems reasonable to me

[bz]

I still do not get how this is supposed to fully work but I assume the shortcut from before remains right. I'd say for me you can add:

OKed: bz