s390/ftrace: fix potential crashes when switching tracers

Switching tracers include instruction patching. To prevent that a
instruction is patched while it's read the instruction patching is done
in stop_machine 'context'. This also means that any function called
during stop_machine must not be traced. Thus add 'notrace' to all
functions called within stop_machine.

Fixes: 1ec2772e ("s390/diag: add a statistic for diagnose calls")
Fixes: 38f2c691 ("s390: improve wait logic of stop_machine")
Fixes: 4ecf0a43 ("processor: get rid of cpu_relax_yield")
Signed-off-by: Philipp Rudo <prudo@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>