nvmet-rdma: Implement get_mdts controller op

Set the maximal data transfer size to be 1MB (currently mdts is
unlimited). This will allow calculating the amount of MR's that
one ctrl should allocate to fulfill it's capabilities.
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Max Gurtovoy <maxg@mellanox.com>

drivers/nvme/target/rdma.c

@@ -31,6 +31,9 @@
 #define NVMET_RDMA_MAX_INLINE_SGE		4
 #define NVMET_RDMA_MAX_INLINE_DATA_SIZE		max_t(int, SZ_16K, PAGE_SIZE)
 
+/* Assume mpsmin == device_page_size == 4KB */
+#define NVMET_RDMA_MAX_MDTS			8
+
 struct nvmet_rdma_cmd {
 	struct ib_sge		sge[NVMET_RDMA_MAX_INLINE_SGE + 1];
 	struct ib_cqe		cqe;
@@ -1602,6 +1605,11 @@ static void nvmet_rdma_disc_port_addr(struct nvmet_req *req,
 	}
 }
 
+static u8 nvmet_rdma_get_mdts(const struct nvmet_ctrl *ctrl)
+{
+	return NVMET_RDMA_MAX_MDTS;
+}
+
 static const struct nvmet_fabrics_ops nvmet_rdma_ops = {
 	.owner			= THIS_MODULE,
 	.type			= NVMF_TRTYPE_RDMA,
@@ -1612,6 +1620,7 @@ static const struct nvmet_fabrics_ops nvmet_rdma_ops = {
 	.queue_response		= nvmet_rdma_queue_response,
 	.delete_ctrl		= nvmet_rdma_delete_ctrl,
 	.disc_traddr		= nvmet_rdma_disc_port_addr,
+	.get_mdts		= nvmet_rdma_get_mdts,
 };
 
 static void nvmet_rdma_remove_one(struct ib_device *ib_device, void *client_data)