KVM: x86/mmu: Don't require refcounted "struct page" to create huge SPTEs

Drop the requirement that a pfn be backed by a refcounted, compound or
or ZONE_DEVICE, struct page, and instead rely solely on the host page
tables to identify huge pages.  The PageCompound() check is a remnant of
an old implementation that identified (well, attempt to identify) huge
pages without walking the host page tables.  The ZONE_DEVICE check was
added as an exception to the PageCompound() requirement.  In other words,
neither check is actually a hard requirement, if the primary has a pfn
backed with a huge page, then KVM can back the pfn with a huge page
regardless of the backing store.

Dropping the @pfn parameter will also allow KVM to query the max host
mapping level without having to first get the pfn, which is advantageous
for use outside of the page fault path where KVM wants to take action if
and only if a page can be mapped huge, i.e. avoids the pfn lookup for
gfns that can't be backed with a huge page.

Cc: Mingwei Zhang <mizhang@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Mingwei Zhang <mizhang@google.com>
Message-Id: <20220715232107.3775620-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>