rtnetlink: s/IFLA_IF_NETNSID/IFLA_TARGET_NETNSID/g

IFLA_TARGET_NETNSID is the new alias for IFLA_IF_NETNSID. This commit
replaces all occurrences of IFLA_IF_NETNSID with the new alias to
indicate that this identifier is the preferred one.

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