server: Deal with jumbo frames on the wire

The 2000 as a number is too small. Modern networks can have a
higher MTU (up to 9000). Take this number and assume there is
a big header in front of it.
diff --git a/src/osmo_server_network.c b/src/osmo_server_network.c
index 78cda94..7127401 100644
--- a/src/osmo_server_network.c
+++ b/src/osmo_server_network.c
@@ -199,7 +199,7 @@
 	} else if (conn->pend == 0) {
 		conn->data->len = ntohs(conn->data->len);
 
-		if (conn->data->len > 2000) {
+		if (conn->data->len > SERVER_MAX_DATA_SIZE) {
 			LOGP(DSERVER, LOGL_ERROR,
 			     "Implausible data length: %u\n", conn->data->len);
 			close_connection(conn);