KVM: selftests: Fix divide-by-zero bug in memslot_perf_test

Check that the number of pages per slot is non-zero in get_max_slots()
prior to computing the remaining number of pages.  clang generates code
that uses an actual DIV for calculating the remaining, which causes a #DE
if the total number of pages is less than the number of slots.

  traps: memslot_perf_te[97611] trap divide error ip:4030c4 sp:7ffd18ae58f0
         error:0 in memslot_perf_test[401000+cb000]

Fixes: a69170c6 ("KVM: selftests: memslot_perf_test: Report optimal memory slots")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20221213001653.3852042-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/memslot_perf_test.c

@@ -265,6 +265,9 @@ static uint64_t get_max_slots(struct vm_data *data, uint32_t host_page_size)
 	slots = data->nslots;
 	while (--slots > 1) {
 		pages_per_slot = mempages / slots;
+		if (!pages_per_slot)
+			continue;
+
 		rempages = mempages % pages_per_slot;
 		if (check_slot_pages(host_page_size, guest_page_size,
 				     pages_per_slot, rempages))