kvm: x86/mmu: Flush TLB before zap_gfn_range releases RCU

Since "KVM: x86/mmu: Zap only TDP MMU leafs in kvm_zap_gfn_range()"
is going to be reverted, it's not going to be true anymore that
the zap-page flow does not free any 'struct kvm_mmu_page'.  Introduce
an early flush before tdp_mmu_zap_leafs() returns, to preserve
bisectability.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/tdp_mmu.c

@@ -941,13 +941,17 @@ static bool tdp_mmu_zap_leafs(struct kvm *kvm, struct kvm_mmu_page *root,
 		flush = true;
 	}
 
-	rcu_read_unlock();
-
 	/*
-	 * Because this flow zaps _only_ leaf SPTEs, the caller doesn't need
-	 * to provide RCU protection as no 'struct kvm_mmu_page' will be freed.
+	 * Need to flush before releasing RCU.  TODO: do it only if intermediate
+	 * page tables were zapped; there is no need to flush under RCU protection
+	 * if no 'struct kvm_mmu_page' is freed.
 	 */
-	return flush;
+	if (flush)
+		kvm_flush_remote_tlbs_with_address(kvm, start, end - start);
+
+	rcu_read_unlock();
+
+	return false;
 }
 
 /*