libiscsi, iser: Adjust data_length to include protection information

In case protection information exists over the wire
iscsi header data length is required to include it.
Use protection information aware scsi helpers to set
the correct transfer length.

In order to avoid breakage, remove iser transfer length
checks for each task as they are not always true and
somewhat redundant anyway.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Reviewed-by: Mike Christie <michaelc@cs.wisc.edu>
Acked-by: Mike Christie <michaelc@cs.wisc.edu>
Cc: stable@vger.kernel.org # 3.15+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>