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>
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 1fa01ff..b59a8d0 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -714,9 +714,7 @@
 
 	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,