ethernet: T5862: default MTU is not acceptable in some environments (backport #3265)

[mergify]

Change Summary

There are cloud environments available where the maximum supported ethernet MTU is e.g. 1450 bytes, thus we clamp this to the adapters maximum MTU value or 1500 bytes - whatever is lower.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

• https://vyos.dev/T5862

Related PR(s)

Component(s) name

Ethernet

How to test

Smoketest result

cpo@LR1.wue3:~$ /usr/libexec/vyos/tests/smoke/cli/test_interfaces_ethernet.py
test_add_multiple_ip_addresses (__main__.EthernetInterfaceTest.test_add_multiple_ip_addresses) ... ok
test_add_single_ip_address (__main__.EthernetInterfaceTest.test_add_single_ip_address) ... ok
test_dhcp_client_options (__main__.EthernetInterfaceTest.test_dhcp_client_options) ... ok
test_dhcp_disable_interface (__main__.EthernetInterfaceTest.test_dhcp_disable_interface) ... ok
test_dhcp_vrf (__main__.EthernetInterfaceTest.test_dhcp_vrf) ... ok
test_dhcpv6_client_options (__main__.EthernetInterfaceTest.test_dhcpv6_client_options) ... ok
test_dhcpv6_vrf (__main__.EthernetInterfaceTest.test_dhcpv6_vrf) ... ok
test_dhcpv6pd_auto_sla_id (__main__.EthernetInterfaceTest.test_dhcpv6pd_auto_sla_id) ... ok
test_dhcpv6pd_manual_sla_id (__main__.EthernetInterfaceTest.test_dhcpv6pd_manual_sla_id) ... ok
test_eapol_support (__main__.EthernetInterfaceTest.test_eapol_support) ... ok
test_ethtool_flow_control (__main__.EthernetInterfaceTest.test_ethtool_flow_control) ... ok
test_ethtool_ring_buffer (__main__.EthernetInterfaceTest.test_ethtool_ring_buffer) ... ok
test_interface_description (__main__.EthernetInterfaceTest.test_interface_description) ... ok
test_interface_disable (__main__.EthernetInterfaceTest.test_interface_disable) ... ok
test_interface_ip_options (__main__.EthernetInterfaceTest.test_interface_ip_options) ... ok
test_interface_ipv6_options (__main__.EthernetInterfaceTest.test_interface_ipv6_options) ... ok
test_interface_mtu (__main__.EthernetInterfaceTest.test_interface_mtu) ... ok
test_ipv6_link_local_address (__main__.EthernetInterfaceTest.test_ipv6_link_local_address) ... ok
test_mtu_1200_no_ipv6_interface (__main__.EthernetInterfaceTest.test_mtu_1200_no_ipv6_interface) ... ok
test_non_existing_interface (__main__.EthernetInterfaceTest.test_non_existing_interface) ... ok
test_offloading_rfs (__main__.EthernetInterfaceTest.test_offloading_rfs) ... ok
test_offloading_rps (__main__.EthernetInterfaceTest.test_offloading_rps) ... ok
test_span_mirror (__main__.EthernetInterfaceTest.test_span_mirror) ... ok
test_speed_duplex_verify (__main__.EthernetInterfaceTest.test_speed_duplex_verify) ... ok
test_vif_8021q_interfaces (__main__.EthernetInterfaceTest.test_vif_8021q_interfaces) ... ok
test_vif_8021q_lower_up_down (__main__.EthernetInterfaceTest.test_vif_8021q_lower_up_down) ... ok
test_vif_8021q_mtu_limits (__main__.EthernetInterfaceTest.test_vif_8021q_mtu_limits) ... ok
test_vif_8021q_qos_change (__main__.EthernetInterfaceTest.test_vif_8021q_qos_change) ... ok
test_vif_s_8021ad_vlan_interfaces (__main__.EthernetInterfaceTest.test_vif_s_8021ad_vlan_interfaces) ... ok
test_vif_s_protocol_change (__main__.EthernetInterfaceTest.test_vif_s_protocol_change) ... ok

----------------------------------------------------------------------
Ran 30 tests in 141.876s

OK

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

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This is an automatic backport of pull request #3265 done by [Mergify](https://mergify.com).