xfrm4: fix the ports decode of sctp protocol

The SCTP pushed the skb data above the sctp chunk header, so the check
of pskb_may_pull(skb, xprth + 4 - skb->data) in _decode_session4() will
never return 0 because xprth + 4 - skb->data < 0, the ports decode of
sctp will always fail.

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>