This patch optimizes zap_threads() for the case when there are no ->mm
users except the current's thread group.  In that case we can avoid
'for_each_process()' loop.

It also adds a useful invariant: SIGNAL_GROUP_EXIT (if checked under
->siglock) always implies that all threads (except may be current) have
pending SIGKILL.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>