[S390] ftrace: dont trace machine check handler

The ftrace code is currently not reentrant, so we better don't trace
our machine check handler. Machine checks are handled like NMIs on s390.

Signed-off-by: Heiko Carstens <[e-mail]>
Signed-off-by: Martin Schwidefsky <[e-mail]>

drivers/s390/s390mach.c

···
 /*
  * machine check handler.
  */
-void
-s390_do_machine_check(struct pt_regs *regs)
+void notrace s390_do_machine_check(struct pt_regs *regs)
 {
 	static DEFINE_SPINLOCK(ipd_lock);
 	static unsigned long long last_ipd;