KVM: Use NULL for struct page pointer to indicate mremapped memory

Drop yet another unnecessary magic page value from KVM, as there's zero
reason to use a poisoned pointer to indicate "no page".  If KVM uses a
NULL page pointer, the kernel will explode just as quickly as if KVM uses
a poisoned pointer.  Never mind the fact that such usage would be a
blatant and egregious KVM bug.

Tested-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20241010182427.1434605-23-seanjc@google.com>