userns: Teach trace to use from_kuid

- When tracing capture the kuid.
- When displaying the data to user space convert the kuid into the
  user namespace of the process that opened the report file.

Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

kernel/trace/trace.h

@@ -147,7 +147,7 @@ struct trace_array_cpu {
 	unsigned long		skipped_entries;
 	cycle_t			preempt_timestamp;
 	pid_t			pid;
-	uid_t			uid;
+	kuid_t			uid;
 	char			comm[TASK_COMM_LEN];
 };