nbd/server: Better error for NBD_OPT_EXPORT_NAME failure

When a client abruptly disconnects before we've finished reading
the name sent with NBD_OPT_EXPORT_NAME, we are better off logging
the failure as EIO (we can't communicate with the client), rather
than EINVAL (the client sent bogus data).
Signed-off-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20180110230825.18321-4-eblake@redhat.com>

nbd/server.c

@@ -299,7 +299,7 @@ static int nbd_negotiate_handle_export_name(NBDClient *client,
     }
     if (nbd_read(client->ioc, name, client->optlen, errp) < 0) {
         error_prepend(errp, "read failed: ");
-        return -EINVAL;
+        return -EIO;
     }
     name[client->optlen] = '\0';
     client->optlen = 0;