xfrm: replace rwlock on xfrm_state_afinfo with rcu

Similar to commit 418a99ac


(Replace rwlock on xfrm_policy_afinfo with rcu), the rwlock
on xfrm_state_afinfo can be replaced by RCU too.

Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>