workqueue: add missing POOL_FREEZING

get_unbound_pool() forgot to set POOL_FREEZING if workqueue_freezing
is set and a new pool could go out of sync with the global freezing
state.

Fix it by adding POOL_FREEZING if workqueue_freezing.  wq_mutex is
already held so no further locking is necessary.  This also removes
the unused static variable warning when !CONFIG_FREEZER.

tj: Updated commit message.

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