netfilter: Drop fragmented ndisc packets assembled in netfilter

netfilter: Drop fragmented ndisc packets assembled in netfilter

Fragmented ndisc packets assembled in netfilter not dropped as specified
in RFC 6980, section 5. This behaviour breaks TAHI IPv6 Core Conformance
Tests v6LC.2.1.22/23, V6LC.2.2.26/27 and V6LC.2.3.18.

Setting IP6SKB_FRAGMENTED flag during reassembly.

References: commit b800c3b966bc ("ipv6: drop fragmented ndisc packets by default (RFC 6980)")
Signed-off-by: Georg Kohmann <geokohma@cisco.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>