kthread_worker: reorganize to prepare for flush_kthread_work() reimplementation

kthread_worker: reorganize to prepare for flush_kthread_work() reimplementation

commit 9a2e03d8ed518a61154f18d83d6466628e519f94 upstream.

Make the following two non-functional changes.

* Separate out insert_kthread_work() from queue_kthread_work().

* Relocate struct kthread_flush_work and kthread_flush_work_fn()
  definitions above flush_kthread_work().

v2: Added lockdep_assert_held() in insert_kthread_work() as suggested
    by Andy Walls.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Andy Walls <awalls@md.metrocast.net>
Cc: Colin Cross <ccross@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>