MINOR: stats/htx: Call channel_add_input instead of updating channel state by hand

This way we are sure the channel state is always correctly upadated, especially
the amount of data directly forwarded. For the stats applet, it is not a bug
because the fast forwarding is never used (the response is chunked and the HTX
extra field is always set to 0).

This patch must be backported to 1.9.
diff --git a/src/stats.c b/src/stats.c
index f69aa90..bb6fa47 100644
--- a/src/stats.c
+++ b/src/stats.c
@@ -245,10 +245,8 @@
 	if (htx) {
 		if (!htx_add_data(htx, ist2(chk->area, chk->data)))
 			return 0;
-		chn->total += chk->data;
+		channel_add_input(chn, chk->data);
 		chk->data = 0;
-		/* notify that some data was read from the SI into the buffer */
-		chn->flags |= CF_READ_PARTIAL;
 	}
 	else  {
 		if (ci_putchk(chn, chk) == -1)