Merge branch 'kvm-no-struct-page' into HEAD

TL;DR: Eliminate KVM's long-standing (and heinous) behavior of essentially
guessing which pfns are refcounted pages (see kvm_pfn_to_refcounted_page()).

Getting there requires "fixing" arch code that isn't obviously broken.
Specifically, to get rid of kvm_pfn_to_refcounted_page(), KVM needs to
stop marking pages/folios dirty/accessed based solely on the pfn that's
stored in KVM's stage-2 page tables.

Instead of tracking which SPTEs correspond to refcounted pages, simply
remove all of the code that operates on "struct page" based ona the pfn
in stage-2 PTEs.  This is the back ~40-50% of the series.

For x86 in particular, which sets accessed/dirty status when that info
would be "lost", e.g. when SPTEs are zapped or KVM clears the dirty flag
in a SPTE, foregoing the updates provides very measurable performance
improvements for related operations.  E.g. when clearing dirty bits as
part of dirty logging, and zapping SPTEs to reconstitue huge pages when
disabling dirty logging.

The front ~40% of the series is cleanups and prep work, and most of it is
x86 focused (purely because x86 added the most special cases, *sigh*).
E.g. several of the inputs to hva_to_pfn() (and it's myriad wrappers),
can be removed by cleaning up and deduplicating x86 code.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>