netns list (just list) is under RTNL. But helper and proto unregistration
happen during rmmod when RTNL is not held, and that's how it was tested:
modprobe/rmmod vs clone(CLONE_NEWNET)/exit.
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com> Signed-off-by: Patrick McHardy <kaber@trash.net> Signed-off-by: David S. Miller <davem@davemloft.net>