KVM: MMU: Fix torn shadow pte

The shadow code assigns a pte directly in one place, which is nonatomic on
i386 can can cause random memory references.  Fix by using an atomic setter.
Signed-off-by: Avi Kivity <avi@qumranet.com>

arch/x86/kvm/paging_tmpl.h

@@ -345,7 +345,7 @@ static u64 *FNAME(fetch)(struct kvm_vcpu *vcpu, gva_t addr,
 		shadow_addr = __pa(shadow_page->spt);
 		shadow_pte = shadow_addr | PT_PRESENT_MASK | PT_ACCESSED_MASK
 			| PT_WRITABLE_MASK | PT_USER_MASK;
-		*shadow_ent = shadow_pte;
+		set_shadow_pte(shadow_ent, shadow_pte);
 	}
 
 	mmu_set_spte(vcpu, shadow_ent, access, walker->pte_access & access,