Fix this sparse warnings:
  arch/x86/xen/smp.c:316:52: warning: Using plain integer as NULL pointer
  arch/x86/xen/smp.c:421:60: warning: Using plain integer as NULL pointer
Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
        BUG_ON(rc);
 
        while(per_cpu(cpu_state, cpu) != CPU_ONLINE) {
-               HYPERVISOR_sched_op(SCHEDOP_yield, 0);
+               HYPERVISOR_sched_op(SCHEDOP_yield, NULL);
                barrier();
        }
 
        /* Make sure other vcpus get a chance to run if they need to. */
        for_each_cpu(cpu, mask) {
                if (xen_vcpu_stolen(cpu)) {
-                       HYPERVISOR_sched_op(SCHEDOP_yield, 0);
+                       HYPERVISOR_sched_op(SCHEDOP_yield, NULL);
                        break;
                }
        }