Before returning NULL, put the sock first.

Cc: stable@vger.kernel.org
Fixes: cf1b2326 ("nbd: verify socket is supported during setup")
Reviewed-by: NJosef Bacik <josef@toxicpanda.com>
Reviewed-by: NMike Christie <mchristi@redhat.com>
Signed-off-by: NSun Ke <sunke32@huawei.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>