SACK broken in HEAD/RELENG_7

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While monitoring a data transfer between two 7-beta3 hosts with tcpdump
and SIFTR[1] last week, we noticed that SACK was not being negotiated as
expected.

The SACK option was sent in the SYN, but the option wasn't sent back in
the SYNACK, despite SACK being enabled on both hosts.

The problem seems to have been caused during some refactoring of
tcp_syncache.c back in march (r1.105), and is fixed by the single line
patch attached.

While investigating the issue, the comments relating to TOF_SACKPERM and
TOF_SACK in tcp_var.h made it a little harder to understand what was
going on. We interpreted the comment for TOF_SACK to imply that it was
used during the handshake. The attached patch only fixes the code, but
it might be worth tweaking these comments as well. Maybe something like:

#define TOF_SACKPERM  0x0004 /* SACK permitted */
#define TOF_SACK  0x0080     /* SACK hole data */

In our opinion, it might also be worth renaming SCF_SACK to SCF_SACKPERM
in tcp_syncache.c to semantically align it with the TOF defines in
tcp_var.h.

James Healy and Lawrence Stewart

http://caia.swin.edu.au

[1] http://caia.swin.edu.au/urp/newtcp/tools.html
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