s390/diag: add a s390 prefix to the diagnose trace point

Documentation/trace/tracepoints.txt states that the naming scheme
for tracepoints is "subsys_event" to avoid collisions. Rename
the 'diagnose' tracepoint to 's390_diagnose'.

Reported-by: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>