KVM: x86/mmu: Get CR4.SMEP from MMU, not vCPU, in shadow page fault

Use the current MMU instead of vCPU state to query CR4.SMEP when handling
a page fault.  In the nested NPT case, the current CR4.SMEP reflects L2,
whereas the page fault is shadowing L1's NPT, which uses L1's hCR4.
Practically speaking, this is a nop a NPT walks are always user faults,
i.e. this code will never be reached, but fix it up for consistency.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622175739.3610207-54-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/paging_tmpl.h

@@ -903,7 +903,7 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gpa_t addr, u32 error_code,
 		 * then we should prevent the kernel from executing it
 		 * if SMEP is enabled.
 		 */
-		if (kvm_read_cr4_bits(vcpu, X86_CR4_SMEP))
+		if (is_cr4_smep(vcpu->arch.mmu))
 			walker.pte_access &= ~ACC_EXEC_MASK;
 	}