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 b800c3b9 ("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>