blk-mq: kill preempt disable/enable in blk_mq_work_fn()

blk_mq_work_fn() is always invoked off the bounded workqueues,
so it can happily preempt among the queues in that set without
causing any issues for blk-mq.
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-mq.c

@@ -714,9 +714,7 @@ static void blk_mq_work_fn(struct work_struct *work)
 
 	hctx = container_of(work, struct blk_mq_hw_ctx, delayed_work.work);
 
-	preempt_disable();
 	__blk_mq_run_hw_queue(hctx);
-	preempt_enable();
 }
 
 static void __blk_mq_insert_request(struct blk_mq_hw_ctx *hctx,