workqueue: fix max_active handling in init_and_link_pwq()

Since 9e8cd2f5 ("workqueue: implement apply_workqueue_attrs()"),
init_and_link_pwq() may be called to initialize a new pool_workqueue
for a workqueue which is already online, but the function was setting
pwq->max_active to wq->saved_max_active without proper
synchronization.

Fix it by calling pwq_adjust_max_active() under proper locking instead
of manually setting max_active.

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