sctp: delete the nested flexible array payload

This patch deletes the flexible-array payload[] from the structure
sctp_datahdr to avoid some sparse warnings:

  # make C=2 CF="-Wflexible-array-nested" M=./net/sctp/
  net/sctp/socket.c: note: in included file (through include/net/sctp/structs.h, include/net/sctp/sctp.h):
  ./include/linux/sctp.h:230:29: warning: nested flexible array

This member is not even used anywhere.
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/sctp.h

@@ -222,7 +222,7 @@ struct sctp_datahdr {
 	__be16 stream;
 	__be16 ssn;
 	__u32 ppid;
-	__u8  payload[];
+	/* __u8  payload[]; */
 };
 
 struct sctp_data_chunk {