ipv6: wire up skb->encapsulation

When pushing a new header before current one call skb_reset_inner_headers
to record the position of the inner headers in the various ipv6 tunnel
protocols.

We later need this to correctly identify the addresses needed to send
back an error in the xfrm layer.

This change is safe, because skb->protocol is always checked before
dereferencing data from the inner protocol.

Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Cc: Nicolas Dichtel <nicolas.dichtel@6wind.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>