[SCTP]: Fix up sctp_rcv return value

I was working on the ipip/xfrm problem and as usual I get side-tracked by
other problems.

As part of an attempt to change the IPv4 protocol handler calling
convention I found that SCTP violated the existing convention.

It's returning non-zero values after freeing the skb.  This is doubly bad
as 1) the skb gets resubmitted; 2) the return value is interpreted as a
protocol number.

This patch changes those return values to zero.

IPv6 doesn't suffer from this problem because it uses a positive return
value as an indication for resubmission.  So the only effect of this patch
there is to increment the IPSTATS_MIB_INDELIVERS counter which IMHO is
the right thing to do.

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