xen/preempt: use need_resched() instead of should_resched()

This code is used only when CONFIG_PREEMPT=n and only in non-atomic
context: xen_in_preemptible_hcall is set only in
privcmd_ioctl_hypercall().  Thus preempt_count is zero and
should_resched() is equal to need_resched().
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

drivers/xen/preempt.c

@@ -31,7 +31,7 @@ EXPORT_SYMBOL_GPL(xen_in_preemptible_hcall);
 asmlinkage __visible void xen_maybe_preempt_hcall(void)
 {
 	if (unlikely(__this_cpu_read(xen_in_preemptible_hcall)
-		     && should_resched())) {
+		     && need_resched())) {
 		/*
 		 * Clear flag as we may be rescheduled on a different
 		 * cpu.