xfrm: ipv6: move mip6_destopt_offset into xfrm core

This helper is relatively small, just move this to the xfrm core
and call it directly.

Next patch does the same for the ROUTING type.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>