MINOR: channel: rename buffer_reserved() to channel_reserved()

This applies to the channel, not the buffer, so let's fix this name.
Warning, the function's name happens to be the same as the old one
which was mistakenly used during 1.5.
diff --git a/include/proto/channel.h b/include/proto/channel.h
index 28ca7c6..1e786f3 100644
--- a/include/proto/channel.h
+++ b/include/proto/channel.h
@@ -300,7 +300,7 @@
  * any chn->size. Special care is taken to avoid any possible integer
  * overflow in the operations.
  */
-static inline int buffer_reserved(const struct channel *chn)
+static inline int channel_reserved(const struct channel *chn)
 {
 	int reserved;
 
@@ -316,7 +316,7 @@
  */
 static inline int buffer_max_len(const struct channel *chn)
 {
-	return chn->buf->size - buffer_reserved(chn);
+	return chn->buf->size - channel_reserved(chn);
 }
 
 /* Returns the amount of space available at the input of the buffer, taking the