portmgrd: Kills swss if MTU is mismatched over Po

[bluecmd]

When adding a port to a portchannel the MTU is checked that it is the same, but if you add this straight to the config or change the MTU later portmgrd will crash and bring down SWSS with it:

Feb 24 09:47:18.037653 ixp-lab-sw1 INFO swss#supervisord: portmgrd RTNETLINK answers: Operation not permitted
Feb 24 09:47:18.037653 ixp-lab-sw1 ERR swss#portmgrd: :- main: Runtime error: /sbin/ip link set dev "Ethernet0" mtu "9216" :·
Feb 24 09:47:18.695593 ixp-lab-sw1 INFO swss#supervisord 2024-02-24 09:47:18,687 INFO exited: portmgrd (exit status 255; not expected)
Feb 24 09:47:18.820290 ixp-lab-sw1 INFO swss#supervisor-proc-exit-listener: Process 'portmgrd' exited unexpectedly. Terminating supervisor 'swss'