tcm_loop: Fix memory leak in tcm_loop_submission_work error path

This patch fixes a tcm_loop_cmd descriptor memory leak in the
tcm_loop_submission_work() error path, and would result in
warnings about leaked tcm_loop_cmd_cache objects at module
unload time.

Go ahead and invoke kmem_cache_free() to release tl_cmd back to
tcm_loop_cmd_cache before calling sc->scsi_done().

Reported-by: Sebastian Herbszt <herbszt@gmx.de>
Tested-by: Sebastian Herbszt <herbszt@gmx.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>