xfrm: ipv6: add xfrm6_hdr_offset helper

This moves the ->hdr_offset indirect call to a new helper.

A followup patch can then modify the new function to replace
the indirect call by direct calls to the required hdr_offset helper.

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