macvtap: advertise link netns via netlink

Assign rtnl_link_ops->get_link_net() callback so that IFLA_LINK_NETNSID is
added to rtnetlink messages. This fixes iproute2 which otherwise resolved
the link interface to an interface in the wrong namespace.

Test commands:

  ip netns add nst
  ip link add dummy0 type dummy
  ip link add link macvtap0 link dummy0 type macvtap
  ip link set macvtap0 netns nst
  ip -netns nst link show macvtap0

Before:

  10: macvtap0@gre0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 500
      link/ether 5e:8f:ae:1d:60:50 brd ff:ff:ff:ff:ff:ff

After:

  10: macvtap0@if2: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 500
      link/ether 5e:8f:ae:1d:60:50 brd ff:ff:ff:ff:ff:ff link-netnsid 0

Reported-by: Leonardo Mörlein <freifunk@irrelefant.net>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Link: https://lore.kernel.org/r/20220228003240.1337426-1-sven@narfation.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>