[S390] idle: Fix machine check handling in idle loop.

If a machine check handling is pending when the idle loop is entered
default_idle will be left with timer ticks and virtual timer disabled.
Fix this by "calling" the idle_chain. Also a BUG_ON(!in_interrupt) in
start_hz_timer must be removed since the function now gets called from
non interrupt context as well.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/process.c

@@ -152,6 +152,10 @@ static void default_idle(void)
 	local_mcck_disable();
 	if (test_thread_flag(TIF_MCCK_PENDING)) {
 		local_mcck_enable();
+		/* disable monitor call class 0 */
+		__ctl_clear_bit(8, 15);
+		atomic_notifier_call_chain(&idle_chain, S390_CPU_NOT_IDLE,
+					   hcpu);
 		local_irq_enable();
 		s390_handle_mcck();
 		return;