ipv6: Don't reset inner headers in ip6_tnl_xmit

Since iptunnel_handle_offloads() is called in all paths we can
probably drop the block in ip6_tnl_xmit that was checking for
skb->encapsulation and resetting the inner headers.

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>