tcp: Revert "tcp: remove CA_ACK_SLOWPATH"

This change was a followup to the header prediction removal,
so first revert this as a prerequisite to back out hp removal.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/tcp.h b/include/net/tcp.h
index c614ff1..c546d13 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -910,8 +910,9 @@ enum tcp_ca_event {
 
 /* Information about inbound ACK, passed to cong_ops->in_ack_event() */
 enum tcp_ca_ack_event_flags {
-	CA_ACK_WIN_UPDATE	= (1 << 0),	/* ACK updated window */
-	CA_ACK_ECE		= (1 << 1),	/* ECE bit is set on ack */
+	CA_ACK_SLOWPATH		= (1 << 0),	/* In slow path processing */
+	CA_ACK_WIN_UPDATE	= (1 << 1),	/* ACK updated window */
+	CA_ACK_ECE		= (1 << 2),	/* ECE bit is set on ack */
 };
 
 /*