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/include/linux/sctp.h b/include/linux/sctp.h
index 09d7412..da803df 100644
--- a/include/linux/sctp.h
+++ b/include/linux/sctp.h
@@ -231,7 +231,7 @@ struct sctp_datahdr {
 	__be32 tsn;
 	__be16 stream;
 	__be16 ssn;
-	__be32 ppid;
+	__u32 ppid;
 	__u8  payload[0];
 };