network/netdev: do not update MAC address if netdev is already running

network/netdev: do not update MAC address if netdev is already running

Follow-up for 17c5337f7b2993619d84acc2088b2ba1789e6477.

Older kernels (older than v6.5) refuse RTM_NEWLINK messages with IFLA_ADDRESS
attribute when the netdev already exists and is running, even if the MAC
address is unchanged.

So, let's not set IFLA_ADDRESS or IFLA_MTU if they are unchanged, and
set the attributes only when we can update them.