xfrm: change secpath_set to return secpath struct, not error value

It can only return 0 (success) or -ENOMEM.
Change return value to a pointer to secpath struct.

This avoids direct access to skb->sp:

err = secpath_set(skb);
if (!err) ..
skb->sp-> ...

Becomes:
sp = secpath_set(skb)
if (!sp) ..
sp-> ..

This reduces noise in followup patch which is going to remove skb->sp.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>