xfrm: remove hdr_offset indirection

After previous patches all remaining users set the function pointer to
the same function: xfrm6_find_1stfragopt.

So remove this function pointer and call ip6_find_1stfragopt directly.

Reduces size of xfrm_type to 64 bytes on 64bit platforms.

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