exit: avoid sig->count in __exit_signal() to detect the group-dead case

Change __exit_signal() to check thread_group_leader() instead of
atomic_dec_and_test(&sig->count).  This must be equivalent, the group
leader must be released only after all other threads have exited and
passed __exit_signal().

Henceforth sig->count is not actually used, except in fs/proc for
get_nr_threads/etc.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Roland McGrath <roland@redhat.com>
Cc: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>