tcp: options clean up

This should fix the following bugs:
  * Connections with MD5 signatures produce invalid packets whenever SACK
    options are included
  * MD5 signatures are counted twice in the MSS calculations

Behaviour changes:
  * A SYN with MD5 + SACK + TS elicits a SYNACK with MD5 + SACK

    This is because we can't fit any SACK blocks in a packet with MD5 + TS
    options. There was discussion about disabling SACK rather than TS in
    order to fit in better with old, buggy kernels, but that was deemed to
    be unnecessary.

  * SYNs with MD5 don't include a TS option

    See above.

Additionally, it removes a bunch of duplicated logic for calculating options,
which should help avoid these sort of issues in the future.
Signed-off-by: Adam Langley <agl@imperialviolet.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -50,6 +50,7 @@ extern atomic_t tcp_orphan_count;
 extern void tcp_time_wait(struct sock *sk, int state, int timeo);
 
 #define MAX_TCP_HEADER	(128 + MAX_HEADER)
+#define MAX_TCP_OPTION_SPACE 40
 
 /* 
  * Never offer a window over 32767 without using window scaling. Some
@@ -184,6 +185,7 @@ extern void tcp_time_wait(struct sock *sk, int state, int timeo);
 #define TCPOLEN_SACK_BASE_ALIGNED	4
 #define TCPOLEN_SACK_PERBLOCK		8
 #define TCPOLEN_MD5SIG_ALIGNED		20
+#define TCPOLEN_MSS_ALIGNED		4
 
 /* Flags in tp->nonagle */
 #define TCP_NAGLE_OFF		1	/* Nagle's algo is disabled */