tracing/function-graph-tracer: append the tracing_graph_flag

Impact: Provide a way to pause the function graph tracer

As suggested by Steven Rostedt, the previous patch that prevented from
spinlock function tracing shouldn't use the raw_spinlock to fix it.
It's much better to follow lockdep with normal spinlock, so this patch
adds a new flag for each task to make the function graph tracer able
to be paused. We also can send an ftrace_printk whithout worrying of
the irrelevant traced spinlock during insertion.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/sched.h

@@ -1379,6 +1379,8 @@ struct task_struct {
 	 * because of depth overrun.
 	 */
 	atomic_t trace_overrun;
+	/* Pause for the tracing */
+	atomic_t tracing_graph_pause;
 #endif
 #ifdef CONFIG_TRACING
 	/* state flags for use by tracers */