linuxkpi: Add more `struct folio`-related functions

[dumbbell]

Summary

The i915 DRM driver started to replace the use of struct page by struct folio in its GEM shmem code in Linux 6.12. linuxkpi were missing a few more functions: kmap_local_folio(), memcpy_to_folio() and offset_in_folio(). They are equivalent of their struct page counterparts.

One difference is that kmap_local_folio() takes an offset argument and the returned address takes this offset into account.

This is part of the update of DRM drivers to Linux 6.12.

Sponsored by: The FreeBSD Foundation

[bz]

I wonder when it'll bite us that we implement folio using page and not page using folio...

sys/compat/linuxkpi/common/include/linux/highmem.h

106

char *vaddr;

as per https://lists.freebsd.org/archives/freebsd-arch/2026-March/001270.html

110

I'd also remove the empty line.

[dumbbell]

Address comments from @bz

[dumbbell]

In D56438#1294101, @bz wrote:

I wonder when it'll bite us that we implement folio using page and not page using folio...

Perhaps this is something that we can revisit with your work on FreeBSD vs. Linux pages? I’m not telling you to do it of course, but I could improve that as part of this task.