netfilter: conntrack: nf_conncount_init is failing with IPv6 disabled

The openvswitch module fails initialization when used in a kernel
without IPv6 enabled. nf_conncount_init() fails because the ct code
unconditionally tries to initialize the netns IPv6 related bit,
regardless of the build option. The change below ignores the IPv6
part if not enabled.

Note that the corresponding _put() function already has this IPv6
configuration check.

Fixes: 11efd5cb ("openvswitch: Support conntrack zone limit")
Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>