IB/isert: add module param to set sg_tablesize for IO cmd

Currently, iser target support max IO size of 16MiB by default. For some
adapters, allocating this amount of resources might reduce the total
number of possible connections that can be created. For those adapters,
it's preferred to reduce the max IO size to be able to create more
connections. Since there is no handshake procedure for max IO size in iser
protocol, set the default max IO size to 1MiB and add a module parameter
for enabling the option to control it for suitable adapters.

Fixes: 317000b9 ("IB/isert: allocate RW ctxs according to max IO size")
Link: https://lore.kernel.org/r/20201019094628.17202-1-mgurtovoy@nvidia.com


Reported-by: Krishnamraju Eraparaju <krishna2@chelsio.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>