KVM: selftests: Silence compiler warning in the kvm_page_table_test

When compiling kvm_page_table_test.c, I get this compiler warning
with gcc 11.2:

kvm_page_table_test.c: In function 'pre_init_before_test':
../../../../tools/include/linux/kernel.h:44:24: warning: comparison of
 distinct pointer types lacks a cast
   44 |         (void) (&_max1 == &_max2);              \
      |                        ^~
kvm_page_table_test.c:281:21: note: in expansion of macro 'max'
  281 |         alignment = max(0x100000, alignment);
      |                     ^~~

Fix it by adjusting the type of the absolute value.
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-Id: <20220414103031.565037-1-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/kvm_page_table_test.c

@@ -278,7 +278,7 @@ static struct kvm_vm *pre_init_before_test(enum vm_guest_mode mode, void *arg)
 	else
 		guest_test_phys_mem = p->phys_offset;
 #ifdef __s390x__
-	alignment = max(0x100000, alignment);
+	alignment = max(0x100000UL, alignment);
 #endif
 	guest_test_phys_mem = align_down(guest_test_phys_mem, alignment);