sctp: Add missing annotation for sctp_transport_walk_start()

Sparse reports a warning at sctp_transport_walk_start()

warning: context imbalance in sctp_transport_walk_start
	- wrong count at exit

The root cause is the missing annotation at sctp_transport_walk_start()
Add the missing __acquires(RCU) annotation
Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/socket.c

@@ -5333,7 +5333,7 @@ int sctp_get_sctp_info(struct sock *sk, struct sctp_association *asoc,
 EXPORT_SYMBOL_GPL(sctp_get_sctp_info);
 
 /* use callback to avoid exporting the core structure */
-void sctp_transport_walk_start(struct rhashtable_iter *iter)
+void sctp_transport_walk_start(struct rhashtable_iter *iter) __acquires(RCU)
 {
 	rhltable_walk_enter(&sctp_transport_hashtable, iter);