nvme/rdma: use max BB size as max_xfer_size

We can't transfer more than the bounce buffer in a single command, so
report that rather than some bogus value.

Change-Id: I39b147916dcc2ee478470917298763a239a6a35a
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>

lib/nvme/nvme_rdma.c

@@ -1467,5 +1467,5 @@ uint32_t
 nvme_rdma_ctrlr_get_max_xfer_size(struct spdk_nvme_ctrlr *ctrlr)
 {
 	/* Todo, which should get from the NVMF target */
-	return 506 * 4096;
+	return NVME_RDMA_RW_BUFFER_SIZE;
 }