KVM: x86/mmu: Set mmio_value to '0' if reserved #PF can't be generated

[ Upstream commit 6129ed877d409037b79866327102c9dc59a302fe ]

Set the mmio_value to '0' instead of simply clearing the present bit to
squash a benign warning in kvm_mmu_set_mmio_spte_mask() that complains
about the mmio_value overlapping the lower GFN mask on systems with 52
bits of PA space.

Opportunistically clean up the code and comments.

Cc: stable@vger.kernel.org
Fixes: d43e2675e96fc ("KVM: x86: only do L1TF workaround on affected processors")
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200527084909.23492-1-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>