workqueue: apply __WQ_ORDERED to create_singlethread_workqueue()

create_singlethread_workqueue() is a compat interface for single
threaded workqueue which maps to ordered workqueue w/ rescuer in the
current implementation.  create_singlethread_workqueue() currently
implemented by invoking alloc_workqueue() w/ appropriate parameters.

8719dceae2f9 ("workqueue: reject adjusting max_active or applying
attrs to ordered workqueues") introduced __WQ_ORDERED to protect
ordered workqueues against dynamic attribute changes which can break
ordering guarantees but forgot to apply it to
create_singlethread_workqueue().  This in itself is okay as nobody
currently uses dynamic attribute change on workqueues created with
create_singlethread_workqueue().

However, 4c16bd327c ("workqueue: implement NUMA affinity for unbound
workqueues") broke singlethreaded guarantee for ordered workqueues
through allocating a separate pool_workqueue on each NUMA node by
default.  A later change 8a2b75384444 ("workqueue: fix ordered
workqueues in NUMA setups") fixed it by allocating only one global
pool_workqueue if __WQ_ORDERED is set.

Combined, the __WQ_ORDERED omission in create_singlethread_workqueue()
became critical breaking its single threadedness and ordering
guarantee.

Let's make create_singlethread_workqueue() wrap
alloc_ordered_workqueue() instead so that it inherits __WQ_ORDERED and
can implicitly track future ordered_workqueue changes.

v2: I missed that __WQ_ORDERED now protects against pwq splitting
    across NUMA nodes and incorrectly described the patch as a
    nice-to-have fix to protect against future dynamic attribute
    usages.  Oleg pointed out that this is actually a critical
    breakage due to 8a2b75384444 ("workqueue: fix ordered workqueues
    in NUMA setups").

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Mike Anderson <mike.anderson@us.ibm.com>
Cc: Oleg Nesterov <onestero@redhat.com>
Cc: Gustavo Luiz Duarte <gduarte@redhat.com>
Cc: Tomas Henzl <thenzl@redhat.com>
Cc: stable@vger.kernel.org
Fixes: 4c16bd327c ("workqueue: implement NUMA affinity for unbound workqueues")

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