sctp: move sent_count to the memory hole in sctp_chunk

Now pahole sctp_chunk, it has 2 memory holes:
   struct sctp_chunk {
	struct list_head           list;
	atomic_t                   refcnt;
	/* XXX 4 bytes hole, try to pack */
	...
	long unsigned int          prsctp_param;
	int                        sent_count;
	/* XXX 4 bytes hole, try to pack */

This patch is to move up sent_count to fill the 1st one and eliminate
the 2nd one.

It's not just another struct compaction, it also fixes the "netperf-
Throughput_Mbps -37.2% regression" issue when overloading the CPU.

Fixes: a6c2f792 ("sctp: implement prsctp TTL policy")
Signed-off-by: NXin Long <lucien.xin@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/sctp/structs.h

@@ -554,6 +554,9 @@ struct sctp_chunk {
 
 	atomic_t refcnt;
 
+	/* How many times this chunk have been sent, for prsctp RTX policy */
+	int sent_count;
+
 	/* This is our link to the per-transport transmitted list.  */
 	struct list_head transmitted_list;
 
@@ -610,9 +613,6 @@ struct sctp_chunk {
 	 */
 	unsigned long prsctp_param;
 
-	/* How many times this chunk have been sent, for prsctp RTX policy */
-	int sent_count;
-
 	/* Which association does this belong to?  */
 	struct sctp_association *asoc;