sctp: move rcu_read_lock from __sctp_lookup_association to sctp_lookup_association

__sctp_lookup_association() is only invoked by sctp_v4_err() and
sctp_rcv(), both which run on the rx BH, and it has been protected
by rcu_read_lock [see ip_local_deliver_finish() / ipv6_rcv()].

So we can move it to sctp_lookup_association, only let
sctp_lookup_association use rcu_read_lock.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/input.c

@@ -937,7 +937,6 @@ static struct sctp_association *__sctp_lookup_association(
 	struct sctp_transport *t;
 	struct sctp_association *asoc = NULL;
 
-	rcu_read_lock();
 	t = sctp_addrs_lookup_transport(net, local, peer);
 	if (!t || !sctp_transport_hold(t))
 		goto out;
@@ -949,7 +948,6 @@ static struct sctp_association *__sctp_lookup_association(
 	sctp_transport_put(t);
 
 out:
-	rcu_read_unlock();
 	return asoc;
 }
 
@@ -962,7 +960,9 @@ struct sctp_association *sctp_lookup_association(struct net *net,
 {
 	struct sctp_association *asoc;
 
+	rcu_read_lock();
 	asoc = __sctp_lookup_association(net, laddr, paddr, transportp);
+	rcu_read_unlock();
 
 	return asoc;
 }