[IP6TUNNEL]: Make the fallback tunnel device per-net.

All the code, that reference it already has the ip6_tnl_net pointer,
so s/ip6_fb_tnl_dev/ip6n->fb_tnl_dev/ and move creation/releasing
code into net init/exit ops.

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