Comment 3 for bug 2047798

It seems to the be the 'protocol' ip implements internally: https://github.com/iproute2/iproute2/blob/f22c49730c3691c25a1147081363eb35aa9d1048/ip/ipnetns.c#L819 which results in a failure in some specific scenarios (when trying to make /run/netns a shared mount point).