workqueue: factor out start_flush_work()

Factor out start_flush_work() from flush_work().  start_flush_work()
has @wait_executing argument which controls whether the barrier is
queued only if the work is pending or also if executing.  As
flush_work() needs to wait for execution too, it uses %true.

This commit doesn't cause any behavior difference.  start_flush_work()
will be used to implement flush_work_sync().

Signed-off-by: Tejun Heo <tj@kernel.org>