KVM: x86/mmu: Use -1 to flag an undefined spte in get_mmio_spte()

Return -1 from the get_walk() helpers if the shadow walk doesn't fill at
least one spte, which can theoretically happen if the walk hits a
not-present PDPTR.  Returning the root level in such a case will cause
get_mmio_spte() to return garbage (uninitialized stack data).  In
practice, such a scenario should be impossible as KVM shouldn't get a
reserved-bit page fault with a not-present PDPTR.

Note, using mmu->root_level in get_walk() is wrong for other reasons,
too, but that's now a moot point.

Fixes: 95fb5b02 ("kvm: x86/mmu: Support MMIO in the TDP MMU")
Cc: Ben Gardon <bgardon@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20201218003139.2167891-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>