ipvlan: advertise link netns via netlink

Assign rtnl_link_ops->get_link_net() callback so that IFLA_LINK_NETNSID is
added to rtnetlink messages.

Test commands:
    ip netns add nst
    ip link add dummy0 type dummy
    ip link add ipvlan0 link dummy0 type ipvlan
    ip link set ipvlan0 netns nst
    ip netns exec nst ip link show ipvlan0

Result:
    ---Before---
    6: ipvlan0@if5: <BROADCAST,MULTICAST> ...
        link/ether 82:3a:78:ab:60:50 brd ff:ff:ff:ff:ff:ff

    ---After---
    12: ipvlan0@if11: <BROADCAST,MULTICAST> ...
        link/ether 42:b1:ad:57:4e:27 brd ff:ff:ff:ff:ff:ff link-netnsid 0
                                                           ~~~~~~~~~~~~~~

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>