target: Rename target_allocate_tasks to target_setup_cmd_from_cdb

This patch renames a horribly misnamed function that no longer allocate
tasks to something more descriptive for it's modern use in target core.

(nab: Fix up ib_srpt to use this as well ahead of a target_submit_cmd
conversion)

Change-Id: Ib7ab2b67723753777084fffe19c47e6eac3b255e
Signed-off-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>