tcp: Make prior_ssthresh a u32

If previous window was above representable values of u16,
strange things will happen if undo with the truncated value
is called for. Alternatively, this could be fixed by some
max trickery but that would limit undoing high-speed undos.

Adds 16-bit hole but there isn't anything to fill it with.
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/tcp.h

@@ -355,7 +355,7 @@ struct tcp_sock {
 	u32	lost_retrans_low;	/* Sent seq after any rxmit (lowest) */
 
 	u16	advmss;		/* Advertised MSS			*/
-	u16	prior_ssthresh; /* ssthresh saved at recovery start	*/
+	u32	prior_ssthresh; /* ssthresh saved at recovery start	*/
 	u32	lost_out;	/* Lost packets			*/
 	u32	sacked_out;	/* SACK'd packets			*/
 	u32	fackets_out;	/* FACK'd packets			*/