tcm_loop: kill tcm_loop_allocate_core_cmd

This function makes little sense as a separate abstraction as it's deeply
interwinded with the control flow of its only caller.  Merged it into
tcm_loop_queuecommand after factoring out a helper to convert the task
attribute representation.

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