workqueue: BUG_ON() a pwq release before workqueue_init()

put_pwq() bounces the release to pwq_release_worker to get out from
under pool->lock. workqueue_init() creates that kthread_worker well
after workqueue_init_early() has built the system workqueues, so a pwq
allocation failing partway through apply_wqattrs_prepare() unwinds
through apply_wqattrs_cleanup() and dereferences a NULL worker.

There is no way back from that. Running the release inline is not an
option, that is what the bounce exists for, and workqueue_init_early()
already BUG_ON()s a system workqueue it failed to allocate.

BUG_ON() the missing worker instead of oopsing inside
kthread_queue_work().

Suggested-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/all/144b3dc986135ebe34f671cdd02bd8aa@kernel.org/
Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Tejun Heo <tj@kernel.org>