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

In commit 93b36cf3425b9b ("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>
diff --git a/net/sctp/ipv6.c b/net/sctp/ipv6.c
index 317e13e..14191ab 100644
--- a/net/sctp/ipv6.c
+++ b/net/sctp/ipv6.c
@@ -172,7 +172,8 @@
 
 	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) {