s390/traps: do not test MONITOR CALL without CONFIG_BUG

tinyconfig fails to boot, because without CONFIG_BUG report_bug()
always returns BUG_TRAP_TYPE_BUG, which causes mc 0,0 in
test_monitor_call() to panic. Fix by skipping the test without
CONFIG_BUG.
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/kernel/traps.c

@@ -277,6 +277,8 @@ static void __init test_monitor_call(void)
 {
 	int val = 1;
 
+	if (!IS_ENABLED(CONFIG_BUG))
+		return;
 	asm volatile(
 		"	mc	0,0\n"
 		"0:	xgr	%0,%0\n"