nbd: client_close on error in nbd_co_client_start

Use client_close() if an error in nbd_co_client_start() occurs instead
of manually inlining parts of it. This fixes an assertion error on the
server side if nbd_negotiate() fails.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Acked-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

nbd/server.c

@@ -1082,8 +1082,7 @@ static coroutine_fn void nbd_co_client_start(void *opaque)
         nbd_export_get(exp);
     }
     if (nbd_negotiate(data)) {
-        shutdown(client->sock, 2);
-        client->close(client);
+        client_close(client);
         goto out;
     }
     qemu_co_mutex_init(&client->send_lock);