s390: don't use flush_scheduled_work()

flush_scheduled_work() is deprecated and scheduled to be removed.

* tape_3590: Create and use tape_3590_wq instead of the system_wq.

* tape_block: Directly flush requeue_task on cleanup instead of using
  flush_scheduled_work().

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: linux390@de.ibm.com
Cc: linux-s390@vger.kernel.org