[IPV6]: Unify and cleanup calls to addrconf_sysctl_register

Currently this call is (ab)used similar to devinet one - it
registers sysctls for devices and for the "default" confs, while
the "all" sysctls are registered separately. But unlike its
devinet brother, the passed inet6_device is needed.

The fix is to make a __addrconf_sysctl_register(), which registers
sysctls for all "devices" we need, including "default" and "all" :)

The original addrconf_sysctl_register() calls the introduced
function, passing the inet6_device, device name and ifindex (to
be used as procname and ctl_name) into it.

Thanks to Herbert again for pointing out, that we can shrink the
argument list to 1 :)

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>