ipv6: drop packets with multiple fragmentation headers

[ Upstream commit f46078cf ]

It is not allowed for an ipv6 packet to contain multiple fragmentation
headers. So discard packets which were already reassembled by
fragmentation logic and send back a parameter problem icmp.

The updates for RFC 6980 will come in later, I have to do a bit more
research here.

Cc: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>