net: sctp: rfc4443: do not report ICMP redirects to user space

Adapt the same behaviour for SCTP as present in TCP for ICMP redirect
messages. For IPv6, RFC4443, section 2.4. says:

...
(e) An ICMPv6 error message MUST NOT be originated as a result of
receiving the following:
...
(e.2) An ICMPv6 redirect message [IPv6-DISC].
...

Therefore, do not report an error to user space, just invoke dst's redirect
callback and leave, same for IPv4 as done in TCP as well. The implication
w/o having this patch could be that the reception of such packets would
generate a poll notification and in worst case it could even tear down the
whole connection. Therefore, stop updating sk_err on redirects.

Reported-by: Duan Jiong <[e-mail]>
Reported-by: Hannes Frederic Sowa <[e-mail]>
Suggested-by: Vlad Yasevich <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/sctp/input.c

···
 		break;
 	case ICMP_REDIRECT:
 		sctp_icmp_redirect(sk, transport, skb);
-		err = 0;
-		break;
+		/* Fall through to out_unlock. */
 	default:
 		goto out_unlock;
 	}

net/sctp/ipv6.c

···
 		break;
 	case NDISC_REDIRECT:
 		sctp_icmp_redirect(sk, transport, skb);
-		break;
+		goto out_unlock;
 	default:
 		break;
 	}