KVM: x86/mmu: Shove refcounted page dependency into host_pfn_mapping_level()

Move the check that restricts mapping huge pages into the guest to pfns
that are backed by refcounted 'struct page' memory into the helper that
actually "requires" a 'struct page', host_pfn_mapping_level().  In
addition to deduplicating code, moving the check to the helper eliminates
the subtle requirement that the caller check that the incoming pfn is
backed by a refcounted struct page, and as an added bonus avoids an extra
pfn_to_page() lookup.

Note, the is_error_noslot_pfn() check in kvm_mmu_hugepage_adjust() needs
to stay where it is, as it guards against dereferencing a NULL memslot in
the kvm_slot_dirty_track_enabled() that follows.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220429010416.2788472-11-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>