tcp: avoid integer overflows in tcp_rcv_space_adjust()

When using large tcp_rmem[2] values (I did tests with 500 MB),
I noticed overflows while computing rcvwin.

Lets fix this before the following patch.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Wei Wang <weiwan@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index ca4a636..4f93f095 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -344,7 +344,7 @@ struct tcp_sock {
 
 /* Receiver queue space */
 	struct {
-		int	space;
+		u32	space;
 		u32	seq;
 		u64	time;
 	} rcvq_space;