nbd: fix incorrect unlock of nbd->sock_lock in sock_shutdown

nbd: fix incorrect unlock of nbd->sock_lock in sock_shutdown

Commit 0eadf37afc250 ("nbd: allow block mq to deal with timeouts")
changed normal usage of nbd->sock_lock to use spin_lock/spin_unlock
rather than the *_irq variants, but it missed this unlock in an
error path.

Found by Coverity, CID 1373871.

Signed-off-by: John W. Linville <linville@tuxdriver.com>
Cc: Josef Bacik <jbacik@fb.com>
Cc: Jens Axboe <axboe@fb.com>
Cc: Markus Pargmann <mpa@pengutronix.de>
Fixes: 0eadf37afc250 ("nbd: allow block mq to deal with timeouts")
Signed-off-by: Jens Axboe <axboe@fb.com>