net: add struct newlink_extras

We'll use this struct in the next patch to set some top level IFLA_ members
that we need for restoring macvlan devices.

travis-ci: success for series starting with [v10,01/11] net: pass the struct nlattrs to dump() functions
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>