KVM: ensure __gfn_to_pfn_memslot initializes *writable

For the kvm_is_error_hva, ubsan complains if the uninitialized writable
is passed to __direct_map, even though the value itself is not used
(__direct_map goes to mmu_set_spte->set_spte->set_mmio_spte but never
looks at that argument).

Ensuring that __gfn_to_pfn_memslot initializes *writable is cheap and
avoids this kind of issue.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/kvm_main.c

@@ -1434,11 +1434,17 @@ kvm_pfn_t __gfn_to_pfn_memslot(struct kvm_memory_slot *slot, gfn_t gfn,
 {
 	unsigned long addr = __gfn_to_hva_many(slot, gfn, NULL, write_fault);
 
-	if (addr == KVM_HVA_ERR_RO_BAD)
+	if (addr == KVM_HVA_ERR_RO_BAD) {
+		if (writable)
+			*writable = false;
 		return KVM_PFN_ERR_RO_FAULT;
+	}
 
-	if (kvm_is_error_hva(addr))
+	if (kvm_is_error_hva(addr)) {
+		if (writable)
+			*writable = false;
 		return KVM_PFN_NOSLOT;
+	}
 
 	/* Do not map writable pfn in the readonly memslot. */
 	if (writable && memslot_is_readonly(slot)) {