ipv6: set skb->protocol on tcp, raw and ip6_append_data genereated skbs

Currently we don't initialize skb->protocol when transmitting data via
tcp, raw(with and without inclhdr) or udp+ufo or appending data directly
to the socket transmit queue (via ip6_append_data). This needs to be
done so that we can get the correct mtu in the xfrm layer.

Setting of skb->protocol happens only in functions where we also have
a transmitting socket and a new skb, so we don't overwrite old values.

Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>