ftrace: show virtual PID

Impact: fix PID output under namespaces

When current namespace is not the global namespace,
pid read from set_ftrace_pid is no correct.

 # ~/newpid_namespace_run bash
 # echo $$
 1
 # echo 1 > set_ftrace_pid
 # cat set_ftrace_pid
 3756

Since we write virtual PID to set_ftrace_pid, we need get
virtual PID when we read it.
Signed-off-by: NLai Jiangshan <laijs@cn.fujitsu.com>
Cc: Steven Rostedt <srostedt@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <49C84D65.9050606@cn.fujitsu.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/trace/ftrace.c

@@ -2264,7 +2264,7 @@ ftrace_pid_read(struct file *file, char __user *ubuf,
 	if (ftrace_pid_trace == ftrace_swapper_pid)
 		r = sprintf(buf, "swapper tasks\n");
 	else if (ftrace_pid_trace)
-		r = sprintf(buf, "%u\n", pid_nr(ftrace_pid_trace));
+		r = sprintf(buf, "%u\n", pid_vnr(ftrace_pid_trace));
 	else
 		r = sprintf(buf, "no pid\n");