[IPV6]: Make ipv6_frag_rcv return the same packet

This patch implements the same change taht was done to ip_defrag.  It
makes ipv6_frag_rcv return the last packet received of a train of fragments
rather than the head of that sequence.

This allows us to get rid of the sk_buff ** argument later.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>