Documentation/target: Fix tcm_mod_builder.py build breakage

This patch fixes TFO->release_cmd() and removes legacy pack_lun() usage
and new_cmd_failure when generating new TCM fabric skeleton from the
tcm_mod_builder.py script.

Reported-by: Stefan Bergstrand <stefan.bergstrand@sdsab.se>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>