net: ipv6: Fix fragmented IPv6 packet sending

Earlier we modified the original pkt chain when creating IPv6
fragments. This is not a proper way as the original chain might
still be used in TCP when re-sending a message. So when fragmenting
the packet, clone it first and leave original packet intact. This
occupies litle more memory but is now safe.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>