exit: exit_notify: re-use "dead" list to autoreap current

exit: exit_notify: re-use "dead" list to autoreap current

After the previous change we can add just the exiting EXIT_DEAD task to
the "dead" list and remove another release_task(tsk).

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Aaron Tomlin <atomlin@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Sterling Alexander <stalexan@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>