target: remove transport_allocate_tasks

There were only two callers, and one of them always wants the call
to transport_allocate_data_tasks anyway.  Also drop the constant
lba argument to transport_allocate_data_tasks and move the variables
inside it into the minimum required scope.

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