workqueue: wake up a worker when a rescuer is leaving a gcwq

After executing the matching works, a rescuer leaves the gcwq whether
there are more pending works or not.  This may decrease the
concurrency level to zero and stall execution until a new work item is
queued on the gcwq.

Make rescuer wake up a regular worker when it leaves a gcwq if there
are more works to execute, so that execution isn't stalled.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Ray Jui <rjui@broadcom.com>
Cc: stable@kernel.org