preparing for shutdown: updates
netdev: netdev_mnl_set_mtu(): drop unneeded variables
Setting an MTU via `RTM_NEWLINK` does not require `ifi_flags`/
`ifi_change`, so we don't really need to set these fields.
Change-Id: I8f9d39733a80a0ec4e8c462f4ae32b4ac52b5849
