[systemd-devel] UseMTU should not be enabled by default in systemd 235

[Andrew Jeddeloh]

I saw in https://github.com/systemd/systemd/pull/6936 that networkd's
UseMTU will be enabled by default in 235. There is an bug
https://github.com/systemd/systemd/issues/6593 that causes networkd to
loop continually setting and unsetting the MTU on most real hardware
if UseMTU is set and the the DHCP lease specifies a larger value than
what the driver is currently using.

This can be reproduced on with the e1000 ethernet driver as well as
ixgbe. See https://github.com/coreos/bugs/issues/1827#issuecomment-321129028
for reproduction setups using a Container Linux image and dnsmasq
locally.

This bug should be fixed before UseMTU is enabled by default.

-Andrew Jeddeloh