nvme-rdma: fix merge error

The merge of 4.12-rc5 into the for-4.13/block tree didn't handle the queue
ready case correctly.  Fix this by propagating blk_status_t into
nvme_rdma_queue_is_ready.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>