Remove a leftover spinlock.

This was required back when mod_delayed_work() did not exist, and had to
be implemented with a cancel + queue. See commit e7c2f967
("workqueue: use mod_delayed_work() instead of __cancel + queue")

schedule_delayed_work() and mod_delayed_work() can now be used
concurrently. So the spinlock is no longer needed.
Signed-off-by: Sven Van Asbroeck <TheSven73@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>