ptrace: don't clear GROUP_STOP_SIGMASK on double-stop

ptrace: don't clear GROUP_STOP_SIGMASK on double-stop

[This does not correspond to any specific patch in the upstream tree as it was
fixed accidentally by rewriting the code in the 3.1 release]

https://bugzilla.redhat.com/show_bug.cgi?id=740121

1. Luke Macken triggered WARN_ON(!(group_stop & GROUP_STOP_SIGMASK))
   in do_signal_stop().

   This is because do_signal_stop() clears GROUP_STOP_SIGMASK part
   unconditionally but doesn't update it if task_is_stopped().

2. Looking at this problem I noticed that WARN_ON_ONCE(!ptrace) is
   not right, a stopped-but-resumed tracee can clone the untraced
   thread in the SIGNAL_STOP_STOPPED group, the new thread can start
   another group-stop.

   Remove this warning, we need more fixes to make it true.

Reported-by: Luke Macken <lmacken@redhat.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>