workqueue: Make flush_workqueue() available again to non GPL modules

workqueue: Make flush_workqueue() available again to non GPL modules

Commit 37b1ef31a568fc02e53587620226e5f3c66454c8 ("workqueue: move
flush_scheduled_work() to workqueue.h") moved the exported non GPL
flush_scheduled_work() from a function to an inline wrapper.
Unfortunately, it directly calls flush_workqueue() which is a GPL function.
This has the effect of changing the licensing requirement for this function
and makes it unavailable to non GPL modules.

See commit ad7b1f841f8a54c6d61ff181451f55b68175e15a ("workqueue: Make
schedule_work() available again to non GPL modules") for precedent.

Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Tejun Heo <tj@kernel.org>