workqueue: Correct declaration of cpu_pwq in struct workqueue_struct

cpu_pwq is used in various percpu functions that expect variable in
__percpu address space. Correct the declaration of cpu_pwq to

struct pool_workqueue __rcu * __percpu *cpu_pwq

to declare the variable as __percpu pointer.

The patch also fixes following sparse errors:

workqueue.c:380:37: warning: duplicate [noderef]
workqueue.c:380:37: error: multiple address spaces given: __rcu & __percpu
workqueue.c:2271:15: error: incompatible types in comparison expression (different address spaces):
workqueue.c:2271:15:    struct pool_workqueue [noderef] __rcu *
workqueue.c:2271:15:    struct pool_workqueue [noderef] __percpu *

and uncovers a couple of exisiting "incorrect type in assignment"
warnings (from __rcu address space), which this patch does not address.

Found by GCC's named address space checks.

There were no changes in the resulting object files.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>