s390/ftrace: fix crashes when switching tracers / add notrace to cpu_relax()

With git commit 4d92f502 ("s390: reintroduce diag 44 calls for
cpu_relax()") I reintroduced a non-trivial cpu_relax() variant on s390.

The difference to the previous variant however is that the new version is
an out-of-line function, which will be traced if function tracing is enabled.

Switching to different tracers includes instruction patching. Therefore this
is done within stop_machine() "context" to prevent that any function tracing
is going on while instructions are being patched.
With the new out-of-line variant of cpu_relax() this is not true anymore,
since cpu_relax() gets called in a busy loop by all waiting cpus within
stop_machine() until function patching is finished.
Therefore cpu_relax() must be marked notrace.

This fixes kernel crashes when frequently switching between "function" and
"function_graph" tracers.

Moving cpu_relax() to a header file again, doesn't work because of header
include order dependencies.
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/processor.c

@@ -18,7 +18,7 @@
 
 static DEFINE_PER_CPU(struct cpuid, cpu_id);
 
-void cpu_relax(void)
+void notrace cpu_relax(void)
 {
 	if (!smp_cpu_mtid && MACHINE_HAS_DIAG44)
 		asm volatile("diag 0,0,0x44");