workqueue: Don't call cpumask_test_cpu() with -1 CPU in wq_update_node_max_active()

For wq_update_node_max_active(), @off_cpu of -1 indicates that no CPU is
going down. The function was incorrectly calling cpumask_test_cpu() with -1
CPU leading to oopses like the following on some archs:

  Unable to handle kernel paging request at virtual address ffff0002100296e0
  ..
  pc : wq_update_node_max_active+0x50/0x1fc
  lr : wq_update_node_max_active+0x1f0/0x1fc
  ...
  Call trace:
    wq_update_node_max_active+0x50/0x1fc
    apply_wqattrs_commit+0xf0/0x114
    apply_workqueue_attrs_locked+0x58/0xa0
    alloc_workqueue+0x5ac/0x774
    workqueue_init_early+0x460/0x540
    start_kernel+0x258/0x684
    __primary_switched+0xb8/0xc0
  Code: 9100a273 35000d01 53067f00 d0016dc1 (f8607a60)
  ---[ end trace 0000000000000000 ]---
  Kernel panic - not syncing: Attempted to kill the idle task!
  ---[ end Kernel panic - not syncing: Attempted to kill the idle task! ]---

Fix it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reported-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Link: http://lkml.kernel.org/r/91eacde0-df99-4d5c-a980-91046f66e612@samsung.com
Fixes: 5797b1c1 ("workqueue: Implement system-wide nr_active enforcement for unbound workqueues")