sctp: Do not leak memory on multiple listen() calls

SCTP permits multiple listen call and on subsequent calls
we leak he memory allocated for the crypto transforms.

Signed-off-by: Vlad Yasevich <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/sctp/socket.c

···
 		goto out;
 
 	/* Allocate HMAC for generating cookie. */
-	if (sctp_hmac_alg) {
+	if (!sctp_sk(sk)->hmac && sctp_hmac_alg) {
 		tfm = crypto_alloc_hash(sctp_hmac_alg, 0, CRYPTO_ALG_ASYNC);
 		if (IS_ERR(tfm)) {
 			if (net_ratelimit()) {
···
 		goto cleanup;
 
 	/* Store away the transform reference. */
-	sctp_sk(sk)->hmac = tfm;
+	if (!sctp_sk(sk)->hmac)
+		sctp_sk(sk)->hmac = tfm;
 out:
 	sctp_release_sock(sk);
 	return err;