netfilter: nf_conntrack_bridge: fix CONFIG_IPV6=y

This patch fixes a few problems with CONFIG_IPV6=y and
CONFIG_NF_CONNTRACK_BRIDGE=m:

In file included from net/netfilter/utils.c:5:
include/linux/netfilter_ipv6.h: In function 'nf_ipv6_br_defrag':
include/linux/netfilter_ipv6.h:110:9: error: implicit declaration of function 'nf_ct_frag6_gather'; did you mean 'nf_ct_attach'? [-Werror=implicit-function-declaration]

And these too:

net/ipv6/netfilter.c:242:2: error: unknown field 'br_defrag' specified in initializer
net/ipv6/netfilter.c:243:2: error: unknown field 'br_fragment' specified in initializer

This patch includes an original chunk from wenxu.

Fixes: 764dd163 ("netfilter: nf_conntrack_bridge: add support for IPv6")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reported-by: Yuehaibing <yuehaibing@huawei.com>
Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: wenxu <wenxu@ucloud.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: wenxu <wenxu@ucloud.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>