Pad the following TCP related structs to allow MFCs of upcoming features/fixes
back to the 8 branch:

tcp_var.h

• struct sackhint
• struct tcpcb
• struct tcpstat

The patch breaks the ABI. Bump __FreeBSD_version to 800102 accordingly. User
space tools that rely on the size of any of these structs (e.g. sockstat) need
to be recompiled.

Reviewed by: rpaulo, sam, andre, rwatson
Approved by: re & mentor (gnn)