KVM: MMU: fix MTRR update

Currently, whenever guest MTRR registers are changed
kvm_mmu_reset_context is called to switch to the new root shadow page
table, however, it's useless since:
1) the cache type is not cached into shadow page's attribute so that
   the original root shadow page will be reused

2) the cache type is set on the last spte, that means we should sync
   the last sptes when MTRR is changed

This patch fixs this issue by drop all the spte in the gfn range which
is being updated by MTRR

Signed-off-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index 72bb33f..398d21c 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -171,4 +171,5 @@
 }
 
 void kvm_mmu_invalidate_zap_all_pages(struct kvm *kvm);
+void kvm_zap_gfn_range(struct kvm *kvm, gfn_t gfn_start, gfn_t gfn_end);
 #endif