POOL_MANAGING_WORKERS is used to ensure that at most one worker takes
the manager role at any given time on a given global_cwq.  Trustee
later hitched on it to assume manager adding blocking wait for the
bit.  As trustee already needed a custom wait mechanism, waiting for
MANAGING_WORKERS was rolled into the same mechanism.

Trustee is scheduled to be removed.  This patch separates out
MANAGING_WORKERS wait into per-pool mutex.  Workers use
mutex_trylock() to test for manager role and trustee uses mutex_lock()
to claim manager roles.

gcwq_claim/release_management() helpers are added to grab and release
manager roles of all pools on a global_cwq.  gcwq_claim_management()
always grabs pool manager mutexes in ascending pool index order and
uses pool index as lockdep subclass.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: "Rafael J. Wysocki" <rjw@sisk.pl>