workqueue: Use TASK_IDLE

Workqueues don't use signals, it (ab)uses TASK_INTERRUPTIBLE to avoid
increasing the loadavg numbers. We've 'recently' introduced TASK_IDLE
for this case:

  80ed87c8

 ("sched/wait: Introduce TASK_NOLOAD and TASK_IDLE")

use it.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Tejun Heo <tj@kernel.org>