if_link: add IFLA_TARGET_NETNSID alias

This adds IFLA_TARGET_NETNSID as an alias for IFLA_IF_NETNSID for
RTM_*LINK requests.
The new name is clearer and also aligns with the newly introduced
IFA_TARGET_NETNSID propert for RTM_*ADDR requests.

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