target/iscsi: Eliminate iscsi_cmd.data_length

Redundant, just use iscsi_cmd->se_cmd.data_length once se_cmd is
initialized, or hdr->data_length before then.

Signed-off-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>