selinux/nlmsg: add RTM_NEWNSID and RTM_GETNSID

These new commands are missing.

Fixes: 0c7aecd4

 ("netns: add rtnl cmd to add and get peer netns ids")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>