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

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

Commit afd887449edf2288e9e8a40d851053cb572b2946 ("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 28f9375af903392e3a6c59924d68c83a2e826f9e ("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>