workqueue: break out gcwq->lock locking from gcwq_claim/release_management_and_[un...

workqueue: break out gcwq->lock locking from gcwq_claim/release_management_and_[un]lock()

Releasing management and unlocking gcwq->lock need to be done
separately for the scheduled fix of a subtle idle worker depletion
issue during CPU_ONLINE.  Break out gcwq->lock handling from these
functions.

This patch doesn't introduce any functional difference.

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