rtnetlink: remove check for IFLA_IF_NETNSID

RTM_NEWLINK supports the IFLA_IF_NETNSID property since
5bb8ed07

 so we should not error out
when it is passed.

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