[SCTP]: Always linearise packet on input

I was looking at a RHEL5 bug report involving Xen and SCTP
(https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=212550).
It turns out that SCTP wasn't written to handle skb fragments at
all.  The absence of any calls to skb_may_pull is testament to
that.

It just so happens that Xen creates fragmented packets more often
than other scenarios (header & data split when going from domU to
dom0).  That's what caused this bug to show up.

Until someone has the time sits down and audits the entire net/sctp
directory, here is a conservative and safe solution that simply
linearises all packets on input.
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sctp/input.c

@@ -135,6 +135,9 @@ int sctp_rcv(struct sk_buff *skb)
 
 	SCTP_INC_STATS_BH(SCTP_MIB_INSCTPPACKS);
 
+	if (skb_linearize(skb))
+		goto discard_it;
+
 	sh = (struct sctphdr *) skb->h.raw;
 
 	/* Pull up the IP and SCTP headers. */