exit: move taskstats_tgid_free() from __exit_signal() to free_signal_struct()

Move taskstats_tgid_free() from __exit_signal() to free_signal_struct().

This way signal->stats never points to nowhere and we can read ->stats
lockless.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: Roland McGrath <roland@redhat.com>
Cc: Veaceslav Falico <vfalico@redhat.com>
Cc: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/exit.c

@@ -144,7 +144,6 @@ static void __exit_signal(struct task_struct *tsk)
 	clear_tsk_thread_flag(tsk,TIF_SIGPENDING);
 	if (group_dead) {
 		flush_sigqueue(&sig->shared_pending);
-		taskstats_tgid_free(sig);
 		tty_kref_put(tty);
 	}
 }

kernel/fork.c

@@ -167,6 +167,7 @@ EXPORT_SYMBOL(free_task);
 
 static inline void free_signal_struct(struct signal_struct *sig)
 {
+	taskstats_tgid_free(sig);
 	kmem_cache_free(signal_cachep, sig);
 }