workqueue: reuse the already calculated pwq in try_to_grab_pending()

try_to_grab_pending() was re-calculating the associated pwq using
get_work_pwq() when it already has it cached in a local varible and
the association can't change.  Reuse the local variable instead.

This doesn't introduce any functional changes.

tj: Updated description.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>