RDMA/efa: Expose maximum TX doorbell batch

The device reports the maximum number of bytes to be written before
ringing the doorbell (zero means unlimited).

This patch queries the max batch size and reports it back to the userspace
library.

Link: https://lore.kernel.org/r/20200722140312.3651-2-galpress@amazon.com
Reviewed-by: Daniel Kranzdorf <dkkranzd@amazon.com>
Reviewed-by: Firas JahJah <firasj@amazon.com>
Signed-off-by: Gal Pressman <galpress@amazon.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

include/uapi/rdma/efa-abi.h

@@ -1,6 +1,6 @@
 /* SPDX-License-Identifier: ((GPL-2.0 WITH Linux-syscall-note) OR BSD-2-Clause) */
 /*
- * Copyright 2018-2019 Amazon.com, Inc. or its affiliates. All rights reserved.
+ * Copyright 2018-2020 Amazon.com, Inc. or its affiliates. All rights reserved.
  */
 
 #ifndef EFA_ABI_USER_H
@@ -31,6 +31,8 @@ struct efa_ibv_alloc_ucontext_resp {
 	__u16 sub_cqs_per_cq;
 	__u16 inline_buf_size;
 	__u32 max_llq_size; /* bytes */
+	__u16 max_tx_batch; /* units of 64 bytes */
+	__u8 reserved_90[6];
 };
 
 struct efa_ibv_alloc_pd_resp {