sctp: fix some type cast warnings introduced since very beginning

These warnings were found by running 'make C=2 M=net/sctp/'.
They are there since very beginning.

Note after this patch, there still one warning left in
sctp_outq_flush():
  sctp_chunk_fail(chunk, SCTP_ERROR_INV_STRM)

Since it has been moved to sctp_stream_outq_migrate on net-next,
to avoid the extra job when merging net-next to net, I will post
the fix for it after the merging is done.

Reported-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/sctp/ipv6.c b/net/sctp/ipv6.c
index 7fe9e1d1..a6dfa86 100644
--- a/net/sctp/ipv6.c
+++ b/net/sctp/ipv6.c
@@ -738,7 +738,7 @@ static int sctp_v6_skb_iif(const struct sk_buff *skb)
 /* Was this packet marked by Explicit Congestion Notification? */
 static int sctp_v6_is_ce(const struct sk_buff *skb)
 {
-	return *((__u32 *)(ipv6_hdr(skb))) & htonl(1 << 20);
+	return *((__u32 *)(ipv6_hdr(skb))) & (__force __u32)htonl(1 << 20);
 }
 
 /* Dump the v6 addr to the seq file. */