target: remove the task_lba field in struct se_task

Now that we don't split commands the lba field in the task is always
equivalent to the one in the CDB, even in cases where we have two tasks
due to a BIDI transfer.  Just refer the the lba in the command instead
of duplicating it in the task.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>