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>