pids: Move the pgrp and session pid pointers from task_struct to signal_struct

To access these fields the code always has to go to group leader so
going to signal struct is no loss and is actually a fundamental simplification.

This saves a little bit of memory by only allocating the pid pointer array
once instead of once for every thread, and even better this removes a
few potential races caused by the fact that group_leader can be changed
by de_thread, while signal_struct can not.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

fs/autofs/autofs_i.h

@@ -18,6 +18,7 @@
 #include <linux/string.h>
 #include <linux/wait.h>
 #include <linux/sched.h>
+#include <linux/sched/signal.h>
 #include <linux/mount.h>
 #include <linux/namei.h>
 #include <linux/uaccess.h>