tracing/function-graph-tracer: Drop the useless nmi protection

The function graph tracer used to have a protection against NMI
while entering a function entry tracing. But this is useless now,
this tracer is reentrant and the ring buffer supports the NMI tracing.
We can then drop this protection.
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>

arch/x86/kernel/ftrace.c

@@ -417,10 +417,6 @@ void prepare_ftrace_return(unsigned long *parent, unsigned long self_addr,
 	unsigned long return_hooker = (unsigned long)
 				&return_to_handler;
 
-	/* Nmi's are currently unsupported */
-	if (unlikely(in_nmi()))
-		return;
-
 	if (unlikely(atomic_read(&current->tracing_graph_pause)))
 		return;