[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: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>