KVM: x86/mmu: Fix RCU usage when atomically zapping SPTEs

Fix a missing rcu_dereference in tdp_mmu_zap_spte_atomic.
Reported-by: Nkernel test robot <lkp@intel.com>
Signed-off-by: NBen Gardon <bgardon@google.com>
Message-Id: <20210315233803.2706477-3-bgardon@google.com>
Reviewed-by: NSean Christopherson <seanjc@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/tdp_mmu.c

@@ -543,7 +543,7 @@ static inline bool tdp_mmu_zap_spte_atomic(struct kvm *kvm,
 	 * here since the SPTE is going from non-present
 	 * to non-present.
 	 */
-	WRITE_ONCE(*iter->sptep, 0);
+	WRITE_ONCE(*rcu_dereference(iter->sptep), 0);
 
 	return true;
 }