[TCP]: Make snd_cwnd_cnt 32-bit

Very little point of having 32-bit snd_cnwd if this is not
32-bit as well, as a number of snd_cwnd incrementation formulas
assume that snd_cwnd_cnt can be at least as large as snd_cwnd.

Whether 32-bit is useful was discussed when e0ef57cc
was made:
  http://marc.info/?l=linux-netdev&m=117218144409825&w=2Signed-off-by: NIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/tcp.h

@@ -315,7 +315,7 @@ struct tcp_sock {
  */
  	u32	snd_ssthresh;	/* Slow start size threshold		*/
  	u32	snd_cwnd;	/* Sending congestion window		*/
- 	u16	snd_cwnd_cnt;	/* Linear increase counter		*/
+	u32	snd_cwnd_cnt;	/* Linear increase counter		*/
 	u32	snd_cwnd_clamp; /* Do not allow snd_cwnd to grow above this */
 	u32	snd_cwnd_used;
 	u32	snd_cwnd_stamp;