x86/kvm/mmu: make vcpu->mmu a pointer to the current MMU

As a preparation to full MMU split between L1 and L2 make vcpu->arch.mmu
a pointer to the currently used mmu. For now, this is always
vcpu->arch.root_mmu. No functional change.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Sean Christopherson <sean.j.christopherson@intel.com>

arch/x86/include/asm/kvm_host.h

@@ -543,7 +543,10 @@ struct kvm_vcpu_arch {
 	 * the paging mode of the l1 guest. This context is always used to
 	 * handle faults.
 	 */
-	struct kvm_mmu mmu;
+	struct kvm_mmu *mmu;
+
+	/* Non-nested MMU for L1 */
+	struct kvm_mmu root_mmu;
 
 	/*
 	 * Paging state of an L2 guest (used for nested npt)