[TCP]: Move sack_ok access to obviously named funcs & cleanup

Previously code had IsReno/IsFack defined as macros that were
local to tcp_input.c though sack_ok field has user elsewhere too
for the same purpose. This changes them to static inlines as
preferred according the current coding style and unifies the
access to sack_ok across multiple files. Magic bitops of sack_ok
for FACK and DSACK are also abstracted to functions with
appropriate names.

Note:
- One sack_ok = 1 remains but that's self explanary, i.e., it
  enables sack
- Couple of !IsReno cases are changed to tcp_is_sack
- There were no users for IsDSack => I dropped it

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -719,6 +719,34 @@ static inline void tcp_ca_event(struct sock *sk, const enum tcp_ca_event event)
 		icsk->icsk_ca_ops->cwnd_event(sk, event);
 }
 
+/* These functions determine how the current flow behaves in respect of SACK
+ * handling. SACK is negotiated with the peer, and therefore it can vary
+ * between different flows.
+ *
+ * tcp_is_sack - SACK enabled
+ * tcp_is_reno - No SACK
+ * tcp_is_fack - FACK enabled, implies SACK enabled
+ */
+static inline int tcp_is_sack(const struct tcp_sock *tp)
+{
+	return tp->rx_opt.sack_ok;
+}
+
+static inline int tcp_is_reno(const struct tcp_sock *tp)
+{
+	return !tcp_is_sack(tp);
+}
+
+static inline int tcp_is_fack(const struct tcp_sock *tp)
+{
+	return tp->rx_opt.sack_ok & 2;
+}
+
+static inline void tcp_enable_fack(struct tcp_sock *tp)
+{
+	tp->rx_opt.sack_ok |= 2;
+}
+
 static inline unsigned int tcp_left_out(const struct tcp_sock *tp)
 {
 	return tp->sacked_out + tp->lost_out;