linuxkpi: Add split_page()

This function is supposed to split large pages into an array of
PAGE_SIZE-sized pages, with correct refcounting. This is apparently
used to allow some drivers to free a part of a large page only.

I don't think we use large pages in linuxkpi. Therefore, this new
function is curently a no-op.

The DRM drivers TTM memory manager started to use it in Linux 6.15.

Reviewed by: bz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D58762