zap_other_threads() sets SIGNAL_GROUP_EXIT at the very start,
do_group_exit() doesn't need to do it.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>