openib: Disable CUDA async by default

Disable async receive for CUDA under OpenIB.  While a performance
optimization, it also causes incorrect results for transfers
larger than the GPUDirect RDMA limit.  This change has been validated
and approved by Akshay.

References #3972

Signed-off-by: Brian Barrett <bbarrett@amazon.com>

opal/mca/btl/openib/btl_openib_mca.c

@@ -573,7 +573,7 @@ int btl_openib_register_mca_params(void)
     CHECK(reg_bool("cuda_async_recv", NULL,
                    "Enable or disable CUDA async recv copies "
                    "(true = async; false = sync)",
-                   true, &mca_btl_openib_component.cuda_async_recv));
+                   false, &mca_btl_openib_component.cuda_async_recv));
     /* Also make the max send size larger for better GPU buffer performance */
     mca_btl_openib_module.super.btl_max_send_size = 128 * 1024;
     /* Turn of message coalescing - not sure if it works with GPU buffers */