nbd:fix memory leak in nbd_get_socket()

Before returning NULL, put the sock first.

Cc: stable@vger.kernel.org
Fixes: cf1b2326

 ("nbd: verify socket is supported during setup")
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Sun Ke <sunke32@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>