IB/iser: Remove iser_reg_data_sg helper function

Open coding it makes the code more readable and simple.

Link: https://lore.kernel.org/r/20220308145546.8372-2-mgurtovoy@nvidia.com


Reviewed-by: Sergey Gorenko <sergeygo@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>