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 <[e-mail]>
Cc: Jiri Benc <[e-mail]>
Cc: Nicolas Dichtel <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/uapi/linux/if_addr.h

···
 	IFA_MULTICAST,
 	IFA_FLAGS,
 	IFA_RT_PRIORITY,  /* u32, priority/metric for prefix route */
+	IFA_TARGET_NETNSID,
 	__IFA_MAX,
 };