IB/iser: Use iser_fr_desc as registration context

After removing the FMR support in iSER, there is only one type of
registration context. Replace the void pointer with the explicit structure
for registration (struct iser_fr_desc).

Link: https://lore.kernel.org/r/20220308145546.8372-3-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>