[IPV6]: Fix raw socket hardware checksum failures

When packets hit raw sockets the csum update isn't done yet, do it manually.
Packets can also reach rawv6_rcv on the output path through
ip6_call_ra_chain, in this case skb->ip_summed is CHECKSUM_NONE and this
codepath isn't executed.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>