target: lock => unlock typo in transport_lun_wait_for_tasks

target_stop_cmd() returns with the lock held and IRQs disabled.  The
intent was to unlock here.  This bug was originally added with:

commit cf572a96
Author: Christoph Hellwig <hch@infradead.org>
Date:   Tue Apr 24 00:25:05 2012 -0400

    target: move the state and execute lists to the command

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>