ors-image-backports: set MTU to 1500 on lo

When using IPv6 between gNB and MME on different
ORS, TCP throughput between phones with MTU 1500
and ORS is very bad.
Setting MTU to 1500 on lo (where the eNB / MME GTP
addresses are) fixes the issue.

playbook/ors_backport_after

-1698059244
+1704717268

playbook/roles/ors-image-backports/files/systemd-lo-network

+[Match]
+Name=lo
+
+[Link]
+MTUBytes=1500

playbook/roles/ors-image-backports/tasks/main.yml

@@ -75,6 +75,11 @@
   - name: Configure /etc/systemd/network/dhcp.network
     copy: src=systemd-dhcp-network dest=/etc/systemd/network/dhcp.network owner=root mode=644
 
+## eNB and MME addresses are on lo interface, using high MTU will result in bad throughput 
+## for TCP when using IPv6 and phones with low MTU
+  - name: Configure /etc/systemd/network/lo.network
+    copy: src=systemd-lo-network dest=/etc/systemd/network/lo.network owner=root mode=644
+
   - name: Create a directory if it does not exist
     file: path=/etc/systemd/system/systemd-networkd-wait-online.service.d state=directory mode=0755