workqueue: Fix flag collision

Commit 0a94efb5 ("workqueue: implicit ordered attribute should be
overridable") introduced a __WQ_ORDERED_EXPLICIT flag but gave it the
same value as __WQ_LEGACY.  I don't believe these were intended to
mean the same thing, so renumber __WQ_ORDERED_EXPLICIT.

Fixes: 0a94efb5 ("workqueue: implicit ordered attribute should be ...")
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: stable@vger.kernel.org # v4.13
Signed-off-by: Tejun Heo <tj@kernel.org>