->group_stop_count and setting TASK_STOPPED/SIGNAL_STOP_STOPPED.

This way the tracing hooks can drop and reacquire the siglock freely and
do any blocking hooks without potential SIGCONT races.

With this patch TASK_STOPPED/SIGNAL_STOP_STOPPED is set only when we know
for sure we are going to schedule() after unlock(siglock).
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>