bsd_acct: plain current->real_parent access is not always safe

This is minor, but dereferencing even current real_parent is not safe on debug
kernels, since the memory, this points to, can be unmapped - RCU protection is
required.

Besides, the tgid field is deprecated and is to be replaced with task_tgid_xxx
call (the 2nd patch), so RCU will be required anyway.

Signed-off-by: Pavel Emelyanov <[e-mail]>
Cc: Oleg Nesterov <[e-mail]>
Cc: "Paul E. McKenney" <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/acct.c

···
 #endif
 #if ACCT_VERSION==3
 	ac.ac_pid = current->tgid;
-	ac.ac_ppid = current->real_parent->tgid;
+	rcu_read_lock();
+	ac.ac_ppid = rcu_dereference(current->real_parent)->tgid;
+	rcu_read_unlock();
 #endif
 
 	spin_lock_irq(&current->sighand->siglock);