RDMA/srp: Add parse function for maximum initiator to target IU size

RDMA/srp: Add parse function for maximum initiator to target IU size

According to SRP specifications 'srp-r16a' and 'srp2r06',
IOControllerProfile attributes for SRP target port include the maximum
initiator to target IU size.

SRP connection daemons, such as srp_daemon, can get the value from the
subnet manager. The SRP connection daemon can pass this value to kernel.

This patch adds a parse function for it.

Upstream commit [1] enables the kernel parameter, 'use_imm_data', by
default. [1] also use (8 * 1024) as the default value for kernel parameter
'max_imm_data'. With those default values, the maximum initiator to target
IU size will be 8260.

In case the SRPT modules, which include the in-tree 'ib_srpt.ko' module,
do not support SRP-2 'immediate data' feature, the default maximum
initiator to target IU size is significantly smaller than 8260. For
'ib_srpt.ko' module, which built from source before [2], the default
maximum initiator to target IU is 2116.

[1] introduces a regression issue for old srp targets with default kernel
parameters, as the connection will be rejected because of a too large
maximum initiator to target IU size.

[1] commit 882981f4a411 ("RDMA/srp: Add support for immediate data")
[2] commit 5dabcd0456d7 ("RDMA/srpt: Add support for immediate data")

Link: https://lore.kernel.org/r/20190927174352.7800-1-honli@redhat.com
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Honggang Li <honli@redhat.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>