datapath: Use new header file net/ipv6_frag.h

datapath: Use new header file net/ipv6_frag.h

Upstream commit:
    commit 70b095c84326640eeacfd69a411db8fc36e8ab1a
    Author: Florian Westphal <fw@strlen.de>
    Date:   Sat Jul 14 01:14:01 2018 +0200

    ipv6: remove dependency of nf_defrag_ipv6 on ipv6 module

    IPV6=m
    DEFRAG_IPV6=m
    CONNTRACK=y yields:

    net/netfilter/nf_conntrack_proto.o: In function `nf_ct_netns_do_get':
    net/netfilter/nf_conntrack_proto.c:802: undefined reference to `nf_defrag_ipv6_enable'
    net/netfilter/nf_conntrack_proto.o:(.rodata+0x640): undefined reference to `nf_conntrack_l4proto_icmpv6'

    Setting DEFRAG_IPV6=y causes undefined references to ip6_rhash_params
    ip6_frag_init and ip6_expire_frag_queue so it would be needed to force
    IPV6=y too.

    This patch gets rid of the 'followup linker error' by removing
    the dependency of ipv6.ko symbols from netfilter ipv6 defrag.

    Shared code is placed into a header, then used from both.

    Signed-off-by: Florian Westphal <fw@strlen.de>
    Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
This patch backports the above upstream patch to OVS.

Cc: Florian Westphal <fw@strlen.de>
Tested-by: Greg Rose <gvrose8192@gmail.com>
Reviewed-by: Greg Rose <gvrose8192@gmail.com>
Acked-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: Yifeng Sun <pkusunyifeng@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>