pids: init_struct_pid.tasks should never see the swapper process

"statically initialize struct pid for swapper" commit 820e45db says:

	Statically initialize a struct pid for the swapper process (pid_t == 0)
	and attach it to init_task.  This is needed so task_pid(), task_pgrp()
	and task_session() interfaces work on the swapper process also.

OK, but:

	- it doesn't make sense to add init_task.pids[].node into
	  init_struct_pid.tasks[], and in fact this just wrong.

	  idle threads are special, they shouldn't be visible on any
	  global list. In particular do_each_pid_task(init_struct_pid)
	  shouldn't see swapper.

	  This is the actual reason why kill(0, SIGKILL) from /sbin/init
	  (which starts with 0,0 special pids) crashes the kernel. The
	  signal sent to pgid/sid == 0 must never see idle threads, even
	  if the previous patch fixed the crash itself.

	- we have other idle threads running on the non-boot CPUs, see
	  the next patch.

Change INIT_STRUCT_PID/INIT_PID_LINK to create the empty/unhashed
hlist_head/hlist_node. Like any other idle thread swapper can never exit,
so detach_pid()->__hlist_del() is not possible, but we could change
INIT_PID_LINK() to set pprev = &next if needed.

All we need is the valid swapper->pids[].pid == &init_struct_pid.

Reported-by: Mathias Krause <mathias.krause@secunet.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Cedric Le Goater <clg@fr.ibm.com>
Cc: Dave Hansen <haveblue@us.ibm.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Herbert Poetzl <herbert@13thfloor.at>
Cc: Mathias Krause <Mathias.Krause@secunet.com>
Acked-by: Roland McGrath <roland@redhat.com>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Cc: Sukadev Bhattiprolu <sukadev@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>