workqueue: Move nr_active handling into helpers

__queue_work(), pwq_dec_nr_in_flight() and wq_adjust_max_active() were
open-coding nr_active handling, which is fine given that the operations are
trivial. However, the planned unbound nr_active update will make them more
complicated, so let's move them into helpers.

- pwq_tryinc_nr_active() is added. It increments nr_active if under
  max_active limit and return a boolean indicating whether inc was
  successful. Note that the function is structured to accommodate future
  changes. __queue_work() is updated to use the new helper.

- pwq_activate_first_inactive() is updated to use pwq_tryinc_nr_active() and
  thus no longer assumes that nr_active is under max_active and returns a
  boolean to indicate whether a work item has been activated.

- wq_adjust_max_active() no longer tests directly whether a work item can be
  activated. Instead, it's updated to use the return value of
  pwq_activate_first_inactive() to tell whether a work item has been
  activated.

- nr_active decrement and activating the first inactive work item is
  factored into pwq_dec_nr_active().

v3: - WARN_ON_ONCE(!WORK_STRUCT_INACTIVE) added to __pwq_activate_work() as
      now we're calling the function unconditionally from
      pwq_activate_first_inactive().

v2: - wq->max_active now uses WRITE/READ_ONCE() as suggested by Lai.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <jiangshanlai@gmail.com>