xfrm: merge dstopt and routing hdroff functions

Both functions are very similar, so merge them into one.

The nexthdr is passed as argument to break the loop in the
ROUTING case, this is the only header type where slightly different
rules apply.

While at it, the merged function is realigned with
ip6_find_1stfragopt().  That function received bug fixes for an infinite
loop, but neither dstopt nor rh parsing functions (copy-pasted from
ip6_find_1stfragopt) were changed.

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