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>

drivers/target/loopback/tcm_loop.c

@@ -239,6 +239,7 @@ static void tcm_loop_submission_work(struct work_struct *work)
 	return;
 
 out_done:
+	kmem_cache_free(tcm_loop_cmd_cache, tl_cmd);
 	sc->scsi_done(sc);
 	return;
 }