job control: Small reorganization of wait_consider_task()

Move EXIT_DEAD test in wait_consider_task() above ptrace check.  As
ptraced tasks can't be EXIT_DEAD, this change doesn't cause any
behavior change.  This is to prepare for further changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Oleg Nesterov <oleg@redhat.com>