netfilter: Define v6ops in !CONFIG_NETFILTER case.

When CONFIG_OPENVSWITCH is set, and CONFIG_NETFILTER is not set, the
openvswitch IPv6 fragmentation handling cannot refer to ipv6_ops because
it isn't defined. Add a dummy version to avoid #ifdefs in source files.

Fixes: 7f8a436e

 "openvswitch: Add conntrack action"
Signed-off-by: Joe Stringer <joestringer@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>