nvmet-loop: fix possible leakage during error flow

During nvme_loop_queue_rq error flow, one must call nvme_cleanup_cmd since
it's symmetric to nvme_setup_cmd.

Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>