KVM: x86/mmu: Set @writable to false for non-visible accesses by L2

Explicitly set @writable to false in try_async_pf() if the GFN->PFN
translation is short-circuited due to the requested GFN not being
visible to L2.

Leaving @writable ('map_writable' in the callers) uninitialized is ok
in that it's never actually consumed, but one has to track it all the
way through set_spte() being short-circuited by set_mmio_spte() to
understand that the uninitialized variable is benign, and relying on
@writable being ignored is an unnecessary risk.  Explicitly setting
@writable also aligns try_async_pf() with __gfn_to_pfn_memslot().

Jim Mattson <jmattson@google.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200415214414.10194-2-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -4090,6 +4090,7 @@ static bool try_async_pf(struct kvm_vcpu *vcpu, bool prefault, gfn_t gfn,
 	 */
 	if (is_guest_mode(vcpu) && !kvm_is_visible_gfn(vcpu->kvm, gfn)) {
 		*pfn = KVM_PFN_NOSLOT;
+		*writable = false;
 		return false;
 	}