ipv6: enable IFA_TARGET_NETNSID for RTM_GETADDR

- Backwards Compatibility:
  If userspace wants to determine whether ipv6 RTM_GETADDR requests
  support the new IFA_TARGET_NETNSID property it should verify that the
  reply includes the IFA_TARGET_NETNSID property. If it does not
  userspace should assume that IFA_TARGET_NETNSID is not supported for
  ipv6 RTM_GETADDR requests on this kernel.
- From what I gather from current userspace tools that make use of
  RTM_GETADDR requests some of them pass down struct ifinfomsg when they
  should actually pass down struct ifaddrmsg. To not break existing
  tools that pass down the wrong struct we will do the same as for
  RTM_GETLINK | NLM_F_DUMP requests and not error out when the
  nlmsg_parse() fails.

- Security:
  Callers must have CAP_NET_ADMIN in the owning user namespace of the
  target network namespace.

Signed-off-by: Christian Brauner <christian@brauner.io>
Signed-off-by: David S. Miller <davem@davemloft.net>