mtd_blkdevs: initialze new->rq in add_mtd_blktrans_dev

Various places expect the request_queue in ->rq.  Initialize it to
avoid NULL pointer derefences.

Fixes: 6966bb92

 ("mtd_blkdevs: use blk_mq_alloc_disk")
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>