MINOR: channel: add chn_sess() helper to retrieve session from channel

Channels already have to know what session they below to. Add a helper
to retrieve this pointer so that we'll use it later.
diff --git a/include/proto/channel.h b/include/proto/channel.h
index 05084bb..9907a51 100644
--- a/include/proto/channel.h
+++ b/include/proto/channel.h
@@ -54,6 +54,15 @@
 int bo_getblk_nc(struct channel *chn, char **blk1, int *len1, char **blk2, int *len2);
 
 
+/* returns a pointer to the session the channel belongs to */
+static inline struct session *chn_sess(const struct channel *chn)
+{
+	if (chn->flags & CF_ISRESP)
+		return LIST_ELEM(chn, struct session *, res);
+	else
+		return LIST_ELEM(chn, struct session *, req);
+}
+
 /* returns a pointer to the stream interface feeding the channel (producer) */
 static inline struct stream_interface *chn_prod(const struct channel *chn)
 {