xfrm: use separated locks to protect pointers of struct xfrm_state_afinfo

afinfo->type_map and afinfo->mode_map deserve separated locks,
they are different things.

We should just take RCU read lock to protect afinfo itself,
but not for the inner pointers.

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>