ipv6: move stub initialization after ipv6 setup completion

ipv6: move stub initialization after ipv6 setup completion

The ipv6 stub pointer is currently initialized before the ipv6
routing subsystem: a 3rd party can access and use such stub
before the routing data is ready.
Moreover, such pointer is not cleared in case of initialization
error, possibly leading to dangling pointers usage.

This change addresses the above moving the stub initialization
at the end of ipv6 init code.

Fixes: 5f81bd2e5d80 ("ipv6: export a stub for IPv6 symbols used by vxlan")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>