target/iscsi: Go back to core allocating data buffer for cmd

We originally changed iscsi to allocate its own buffers just as an
intermediate step to clean up some core buffer allocation mechanisms. Now
we can put it back.

Also had to change allocate_iovecs to use data_length instead of
t_data_nents because iovecs are now allocated before the data buffer, thus
t_data_nents is not yet initialized.

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