tcp: Restore 6 tcps padding entries in HEAD

The padding in CURRENT shall not shrink. It is
designed that in CURRENT at always stays
the same, and then when a new stable is branched, it
inherits 6 pointer placeholders that can be used
withing this stable/X lifetime to extend the structure.

Reviewed By: tuexen, #transport
Sponsored by:        NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D34269

sys/netinet/tcp_var.h

@@ -826,7 +826,7 @@ struct	tcpstat {
 	uint64_t tcps_ace_ect0;		/* ACE SYN packet with ECT0 */
 	uint64_t tcps_ace_ce;		/* ACE SYN packet with CE */
 
-	uint64_t _pad[2];		/* 2 TBD */
+	uint64_t _pad[6];		/* 6 TBD placeholder for STABLE */
 };
 
 #define	tcps_rcvmemdrop	tcps_rcvreassfull	/* compat */