macvlan: enforce a consistent minimal mtu

In the Linux kernel, the following vulnerability has been resolved:

macvlan: enforce a consistent minimal mtu

macvlan should enforce a minimal mtu of 68, even at link creation.

This patch avoids the current behavior (which could lead to crashes

in ipv6 stack if the link is brought up)

$ ip link add macvlan1 link eno1 mtu 8 type macvlan # This should fail !

$ ip link sh dev macvlan1

5: macvlan1@eno1: <BROADCAST,MULTICAST> mtu 8 qdisc noop

state DOWN mode DEFAULT group default qlen 1000

link/ether 02:47:6c:24:74:82 brd ff:ff:ff:ff:ff:ff

$ ip link set macvlan1 mtu 67

Error: mtu less than device minimum.

$ ip link set macvlan1 mtu 68

$ ip link set macvlan1 mtu 8

Error: mtu less than device minimum.