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