netns: fix NLM_F_ECHO mechanism for RTM_NEWNSID

The flag NLM_F_ECHO aims to reply to the user the message notified to all
listeners.
It was not the case with the command RTM_NEWNSID, let's fix this.

Fixes: 0c7aecd4 ("netns: add rtnl cmd to add and get peer netns ids")
Reported-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Guillaume Nault <gnault@redhat.com>
Tested-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>