tcp: convert retransmit_cnt_hint to seqno

Main benefit in this is that we can then freely point
the retransmit_skb_hint to anywhere we want to because
there's no longer need to know what would be the count
changes involve, and since this is really used only as a
terminator, unnecessary work is one time walk at most,
and if some retransmissions are necessary after that
point later on, the walk is not full waste of time
anyway.

Since retransmit_high must be kept valid, all lost
markers must ensure that.

Now I also have learned how those "holes" in the
rexmittable skbs can appear, mtu probe does them. So
I removed the misleading comment as well.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/tcp.h

@@ -358,7 +358,7 @@ struct tcp_sock {
 					 */
 
 	int     lost_cnt_hint;
-	int     retransmit_cnt_hint;
+	u32     retransmit_high;	/* L-bits may be on up to this seqno */
 
 	u32	lost_retrans_low;	/* Sent seq after any rxmit (lowest) */