xfrm: state: use atomic_inc_not_zero to increment refcount

Once xfrm_state_lookup_byaddr no longer acquires the state lock another
cpu might be freeing the state entry at the same time.

To detect this we use atomic_inc_not_zero, we then signal -EAGAIN to
caller in case our result was stale.

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