KVM: Don't bother the mmu if cr3 load doesn't change cr3

If the guest requests just a tlb flush, don't take the vm lock and
drop the mmu context pointlessly.

Signed-off-by: Avi Kivity <avi@qumranet.com>
diff --git a/drivers/kvm/x86.h b/drivers/kvm/x86.h
index 71f2477..b1528c9 100644
--- a/drivers/kvm/x86.h
+++ b/drivers/kvm/x86.h
@@ -299,6 +299,7 @@
 
 unsigned long segment_base(u16 selector);
 
+void kvm_mmu_flush_tlb(struct kvm_vcpu *vcpu);
 void kvm_mmu_pte_write(struct kvm_vcpu *vcpu, gpa_t gpa,
 		       const u8 *new, int bytes);
 int kvm_mmu_unprotect_page_virt(struct kvm_vcpu *vcpu, gva_t gva);