[SCTP]: Prevent OOPS if hmac modules didn't load

SCTP was checking for NULL when trying to detect hmac
allocation failure where it should have been using IS_ERR.
Also, print a rate limited warning to the log telling the
user what happend.
Signed-off-by: NVlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sctp/socket.c

@@ -5227,7 +5227,12 @@ int sctp_inet_listen(struct socket *sock, int backlog)
 	/* Allocate HMAC for generating cookie. */
 	if (sctp_hmac_alg) {
 		tfm = crypto_alloc_hash(sctp_hmac_alg, 0, CRYPTO_ALG_ASYNC);
-		if (!tfm) {
+		if (IS_ERR(tfm)) {
+			if (net_ratelimit()) {
+				printk(KERN_INFO
+				       "SCTP: failed to load transform for %s: %ld\n",
+					sctp_hmac_alg, PTR_ERR(tfm));
+			}
 			err = -ENOSYS;
 			goto out;
 		}