if_addr: add IFA_TARGET_NETNSID

This adds a new IFA_TARGET_NETNSID property to be used by address
families such as PF_INET and PF_INET6.
The IFA_TARGET_NETNSID property can be used to send a network namespace
identifier as part of a request. If a IFA_TARGET_NETNSID property is
identified it will be used to retrieve the target network namespace in
which the request is to be made.

Signed-off-by: Christian Brauner <christian@brauner.io>
Cc: Jiri Benc <jbenc@redhat.com>
Cc: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>