linuxkpi: Add more struct folio-related functions

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.

Reviewed by: bz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D56438