iser-target: Fix variable-length response error completion

Since commit "2426bd45 target: Report correct response ..."
we might get a command with data_size that does not fit to
the number of allocated data sg elements. Given that we rely on
cmd t_data_nents which might be different than the data_size,
we sometimes receive local length error completion. The correct
approach would be to take the command data_size into account when
constructing the ib sg_list.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Jenny Falkovich <jennyf@mellanox.com>
Cc: stable@vger.kernel.org # 3.16+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>