workqueue: make GCWQ_DISASSOCIATED a pool flag

Make GCWQ_DISASSOCIATED a pool flag POOL_DISASSOCIATED.  This patch
doesn't change locking - DISASSOCIATED on both pools of a CPU are set
or clear together while holding gcwq->lock.  It shouldn't cause any
functional difference.

This is part of an effort to remove global_cwq and make worker_pool
the top level abstraction, which in turn will help implementing worker
pools with user-specified attributes.

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