device: downgrade warning about IPv6 MTU if IPv6 is disabled
If IPv6 is disabled, changing the IPv6 MTU fails and NM complains with a warning. Since this error is expected and doesn't do any harm, downgrade the logging level to DEBUG.
Since IPv6 kernel support can be built as a module, we have to check the existence of /proc/sys/net/ipv6 every time. Instead of checking it and then setting the MTU (adding one /proc access for everyone), just try to set the MTU; in case of failure, determine the reason for the error.