RDMA/srp: Calculate max_it_iu_size if remote max_it_iu length available

RDMA/srp: Calculate max_it_iu_size if remote max_it_iu length available

The default maximum immediate size is too big for old srp clients, which
do not support immediate data.

According to the SRP and SRP-2 specifications, the IOControllerProfile
attributes for SRP target ports contains the maximum initiator to target
iu length.

The maximum initiator to target iu length can be obtained by sending MAD
packets to query subnet manager port and SRP target ports. We should
calculate the max_it_iu_size instead of the default value, when remote
maximum initiator to target iu length available.

Link: https://lore.kernel.org/r/20190927174352.7800-2-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>