MINOR: channel: simplify the channel's buffer allocation

The channel's buffer allocator, channel_alloc_buffer(), was still relying
on the principle of a margin for the request and not for the response.
But this margin stopped working around 1.7 with the introduction of the
content filters such as SPOE, and was completely anihilated with the
local pools that came with threads. Let's simplify this and just use
b_alloc().
diff --git a/include/haproxy/channel.h b/include/haproxy/channel.h
index 296e465..d37eec4 100644
--- a/include/haproxy/channel.h
+++ b/include/haproxy/channel.h
@@ -832,23 +832,15 @@
 	return end - ci_tail(chn);
 }
 
-/* Allocates a buffer for channel <chn>, but only if it's guaranteed that it's
- * not the last available buffer or it's the response buffer. Unless the buffer
- * is the response buffer, an extra control is made so that we always keep
- * <tune.buffers.reserved> buffers available after this allocation. Returns 0 in
- * case of failure, non-zero otherwise.
+/* Allocates a buffer for channel <chn>. Returns 0 in case of failure, non-zero
+ * otherwise.
  *
  * If no buffer are available, the requester, represented by <wait> pointer,
  * will be added in the list of objects waiting for an available buffer.
  */
 static inline int channel_alloc_buffer(struct channel *chn, struct buffer_wait *wait)
 {
-	int margin = 0;
-
-	if (!(chn->flags & CF_ISRESP))
-		margin = global.tune.reserved_bufs;
-
-	if (b_alloc_margin(&chn->buf, margin) != NULL)
+	if (b_alloc(&chn->buf) != NULL)
 		return 1;
 
 	if (!LIST_ADDED(&wait->list))