blk-mq: remember to start timeout handler for direct queue

blk-mq: remember to start timeout handler for direct queue

Commit b7f56b560d added a direct-to-hw-queue mode, but this mode
needs to remember to add the request timeout handler as well.
Without it, we don't track timeouts for these requests.

Signed-off-by: Jens Axboe <axboe@fb.com>