[NETFILTER]: nf_conntrack: attach conntrack to locally generated ICMPv6 error

Locally generated ICMPv6 errors should be associated with the conntrack
of the original packet. Since the conntrack entry may not be in the hash
tables (for the first packet), it must be manually attached.

Signed-off-by: Yasuyuki Kozakai <yasuyuki.kozakai@toshiba.co.jp>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>