signals: annotate lock context change on ptrace_stop()

ptrace_stop() releases and regrabs current->sighand->siglock but was
missing proper annotation.  Add it.
Signed-off-by: NNamhyung Kim <namhyung@gmail.com>
Acked-by: NRoland McGrath <roland@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/signal.c

@@ -1618,6 +1618,8 @@ static int sigkill_pending(struct task_struct *tsk)
  * is gone, we keep current->exit_code unless clear_code.
  */
 static void ptrace_stop(int exit_code, int clear_code, siginfo_t *info)
+	__releases(&current->sighand->siglock)
+	__acquires(&current->sighand->siglock)
 {
 	if (arch_ptrace_stop_needed(exit_code, info)) {
 		/*