xfrm: ipv6: move mip6_rthdr_offset into xfrm core

Place the call into the xfrm core.  After this all remaining users
set the hdr_offset function pointer to the same function which opens
the possiblity to remove the indirection.

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