xfrm: state lookup can be lockless

This is called from the packet input path, we get lock contention
if many cpus handle ipsec in parallel.

After recent rcu conversion it is safe to call __xfrm_state_lookup
without the spinlock.

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