tracing: Warn on output if the function tracer was found corrupted

As the function tracer is very intrusive, lots of self checks are
performed on the tracer and if something is found to be strange
it will shut itself down keeping it from corrupting the rest of the
kernel. This shutdown may still allow functions to be traced, as the
tracing only stops new modifications from happening. Trying to stop
the function tracer itself can cause more harm as it requires code
modification.

Although a WARN_ON() is executed, a user may not notice it. To help
the user see that something isn't right with the tracing of the system
a big warning is added to the output of the tracer that lets the user
know that their data may be incomplete.

Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/ftrace.c

@@ -3862,6 +3862,14 @@ void ftrace_kill(void)
 	clear_ftrace_function();
 }
 
+/**
+ * Test if ftrace is dead or not.
+ */
+int ftrace_is_dead(void)
+{
+	return ftrace_disabled;
+}
+
 /**
  * register_ftrace_function - register a function for profiling
  * @ops - ops structure that holds the function for profiling.