netns: return RTM_NEWNSID instead of RTM_GETNSID on a get

Usually, RTM_NEWxxx is returned on a get (same as a dump).

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>