nvme-rdma: avoid request double completion for concurrent nvme_rdma_timeout

A crash happens when inject completing request long time(nearly 30s).
Each name space has a request queue, when inject completing request long
time, multi request queues may have time out requests at the same time,
nvme_rdma_timeout will execute concurrently. Multi requests in different
request queues may be queued in the same rdma queue, multi
nvme_rdma_timeout may call nvme_rdma_stop_queue at the same time.
The first nvme_rdma_timeout will clear NVME_RDMA_Q_LIVE and continue
stopping the rdma queue(drain qp), but the others check NVME_RDMA_Q_LIVE
is already cleared, and then directly complete the requests, complete
request before the qp is fully drained may lead to a use-after-free
condition.

Add a multex lock to serialize nvme_rdma_stop_queue.

Signed-off-by: Chao Leng <lengchao@huawei.com>
Tested-by: Israel Rukshin <israelr@nvidia.com>
Reviewed-by: Israel Rukshin <israelr@nvidia.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>