netfilter: fix nf_conntrack_bridge/ipv6 link error

When CONFIG_IPV6 is disabled, the bridge netfilter code
produces a link error:

ERROR: "br_ip6_fragment" [net/bridge/netfilter/nf_conntrack_bridge.ko] undefined!
ERROR: "nf_ct_frag6_gather" [net/bridge/netfilter/nf_conntrack_bridge.ko] undefined!

The problem is that it assumes that whenever IPV6 is not a loadable
module, we can call the functions direction. This is clearly
not true when IPV6 is disabled.

There are two other functions defined like this in linux/netfilter_ipv6.h,
so change them all the same way.

Fixes: 764dd163 ("netfilter: nf_conntrack_bridge: add support for IPv6")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>