tcp: change ICSK_CA_PRIV_SIZE definition

Instead of a magic number (13 currently) and having
to change it every other year, use sizeof_field() macro.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/inet_connection_sock.h

@@ -135,7 +135,7 @@ struct inet_connection_sock {
 	u32			  icsk_user_timeout;
 
 	u64			  icsk_ca_priv[104 / sizeof(u64)];
-#define ICSK_CA_PRIV_SIZE      (13 * sizeof(u64))
+#define ICSK_CA_PRIV_SIZE	  sizeof_field(struct inet_connection_sock, icsk_ca_priv)
 };
 
 #define ICSK_TIME_RETRANS	1	/* Retransmit timer */