net: split do_rtm_link_req into two parts

For macvlan we need to restore in different ways depending on whether we're
inside or outside a user namespace. We want to share the code that does the
building of the base request, so let's split it out into a populate()
function.

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>