IB/isert: Align target max I/O size to initiator size

Since the Linux iser initiator default max I/O size set to 512KB and since
there is no handshake procedure for this size in iser protocol, set the
default max IO size of the target to 512KB as well.

For changing the default values, there is a module parameter for both
drivers.

Link: https://lore.kernel.org/r/20210524085215.29005-1-mgurtovoy@nvidia.com
Reviewed-by: Alaa Hleihel <alaa@nvidia.com>
Reviewed-by: Israel Rukshin <israelr@nvidia.com>
Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Acked-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>