workqueue: implement get/put_pwq()

Add pool_workqueue->refcnt along with get/put_pwq().  Both per-cpu and
unbound pwqs have refcnts and any work item inserted on a pwq
increments the refcnt which is dropped when the work item finishes.

For per-cpu pwqs the base ref is never dropped and destroy_workqueue()
frees the pwqs as before.  For unbound ones, destroy_workqueue()
simply drops the base ref on the first pwq.  When the refcnt reaches
zero, pwq_unbound_release_workfn() is scheduled on system_wq, which
unlinks the pwq, puts the associated pool and frees the pwq and wq as
necessary.  This needs to be done from a work item as put_pwq() needs
to be protected by pool->lock but release can't happen with the lock
held - e.g. put_unbound_pool() involves blocking operations.

Unbound pool->locks are marked with lockdep subclas 1 as put_pwq()
will schedule the release work item on system_wq while holding the
unbound pool's lock and triggers recursive locking warning spuriously.

This will be used to implement dynamic creation and destruction of
unbound pwqs.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>