workqueue: rename workqueue_lock to wq_mayday_lock

With the recent locking updates, the only thing protected by
workqueue_lock is workqueue->maydays list.  Rename workqueue_lock to
wq_mayday_lock.

This patch is pure rename.

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

diff --cc kernel/workqueue.c
Simple merge