ftrace: use struct pid

Impact: clean up, extend PID filtering to PID namespaces

Eric Biederman suggested using the struct pid for filtering on
pids in the kernel. This patch is based off of a demonstration
of an implementation that Eric sent me in an email.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 95fff37..8b81b4d 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -541,11 +541,11 @@
 }
 #endif /* CONFIG_FUNCTION_GRAPH_TRACER */
 
-extern int ftrace_pid_trace;
+extern struct pid *ftrace_pid_trace;
 
 static inline int ftrace_trace_task(struct task_struct *task)
 {
-	if (ftrace_pid_trace < 0)
+	if (ftrace_pid_trace)
 		return 1;
 
 	return test_tsk_trace_trace(task);