network: set ipv6 mtu after link-up or device mtu change

The kernel resets the ipv6 mtu after NETDEV_UP or NETDEV_CHANGEMTU event,
so we must reset the ipv6 mtu to our configured value after we detect
IFF_UP flag set or after we set the device mtu.

Fixes: #13914.
(cherry picked from commit d236718c)
(cherry picked from commit 639dc9f4)