IB/iser: Generalize iser_unmap_task_data and finalize_rdma_unaligned_sg

This routines operates on data buffers and may also work with
protection infomation buffers.  So we generalize them to handle an
iser_data_buf which can be the command data or command protection
information.

This patch does not change any functionality.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>