KVM: Don't reset mmu context unnecessarily when updating EFER

The only bit of EFER that affects the mmu is NX, and this is already
accounted for (LME only takes effect when changing cr0).

Based on a patch by Hillf Danton.
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/kvm/x86.c

@@ -830,7 +830,6 @@ static int set_efer(struct kvm_vcpu *vcpu, u64 efer)
 	kvm_x86_ops->set_efer(vcpu, efer);
 
 	vcpu->arch.mmu.base_role.nxe = (efer & EFER_NX) && !tdp_enabled;
-	kvm_mmu_reset_context(vcpu);
 
 	/* Update reserved bits */
 	if ((efer ^ old_efer) & EFER_NX)