[S390] ftrace: drop nmi protection

The function graph tracer used to have a protection against NMI
while entering a function entry tracing. But this is useless now,
the tracer is reentrant and the ring buffer supports NMI tracing.

Same as 07868b086cca784f4b532fc2ab574ec3a73b468a for x86.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/ftrace.c

@@ -185,9 +185,6 @@ unsigned long prepare_ftrace_return(unsigned long ip, unsigned long parent)
 {
 	struct ftrace_graph_ent trace;
 
-	/* Nmi's are currently unsupported. */
-	if (unlikely(in_nmi()))
-		goto out;
 	if (unlikely(atomic_read(&current->tracing_graph_pause)))
 		goto out;
 	if (ftrace_push_return_trace(parent, ip, &trace.depth, 0) == -EBUSY)