[NETNS]: Clean out the ipv6-related sysctls creation/destruction

The addrconf sysctls and neigh sysctls are registered and
unregistered always in pairs, so they can be joined into
one (well, two) functions, that accept the struct inet6_dev
and do all the job.

This also get rids of unneeded ifdefs inside the code.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>