iser_reg_rdma_mem_[fastreg|fmr] share a lot of code, and
logically do the same thing other than the buffer registration
method itself (iser_fast_reg_mr vs. iser_fast_reg_fmr).
The DIF logic is not implemented in the FMR flow as there is no
existing device that supports FMRs and Signature feature.
This patch unifies the flow in a single routine iser_reg_rdma_mem
and just split to fmr/frwr for the buffer registration itself.
Also, for symmetry reasons, unify iser_unreg_rdma_mem (which will
call the relevant device specific unreg routine).