[IPV6]: Repair IPv6 Fragments

The commit "[IPV6]: Use kmemdup" (commit-id:
af879cc7) broke IPv6 fragments.

Bug was spotted by Yasuyuki Kozakai <yasuyuki.kozakai@toshiba.co.jp>.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>