ipv6: move ip6_sk_accept_pmtu from generic pmtu update path to ipv6 one

In commit 93b36cf ("ipv6: support IPV6_PMTU_INTERFACE on sockets")
I made a horrible mistake to add ip6_sk_accept_pmtu to the generic
sctp_icmp_frag_needed path. This results in build warnings if IPv6 is
disabled which were luckily caught by Fengguang's kbuild bot. But it
also leads to a kernel panic IPv4 frag-needed packet is received.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Acked-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/input.c

@@ -389,9 +389,6 @@ void sctp_icmp_frag_needed(struct sock *sk, struct sctp_association *asoc,
 	if (!t || (t->pathmtu <= pmtu))
 		return;
 
-	if (!ip6_sk_accept_pmtu(sk))
-		return;
-
 	if (sock_owned_by_user(sk)) {
 		asoc->pmtu_pending = 1;
 		t->pmtu_pending = 1;

net/sctp/ipv6.c

@@ -172,7 +172,8 @@ static void sctp_v6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 
 	switch (type) {
 	case ICMPV6_PKT_TOOBIG:
-		sctp_icmp_frag_needed(sk, asoc, transport, ntohl(info));
+		if (ip6_sk_accept_pmtu(sk))
+			sctp_icmp_frag_needed(sk, asoc, transport, ntohl(info));
 		goto out_unlock;
 	case ICMPV6_PARAMPROB:
 		if (ICMPV6_UNK_NEXTHDR == code) {