nbd: enable replace socket if only one connection is configured

Nbd server with multiple connections could be upgraded since
560bc4b3 (nbd: handle dead connections). But if only one conncection
is configured, after we take down nbd server, all inflight IO
would finally timeout and return error. We could requeue them
like what we do with multiple connections and wait for new socket
in submit path.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Hou Pu <houpu@bytedance.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>