MINOR: buffer: add br_single() to check if a buffer ring has more than one buf

It's cheaper and cleaner than using br_count()==1 given that it just compares
two indexes, and that a ring having a single buffer is in a special case where
it is between empty and used up-to-1. In other words it's not congested.

include/haproxy/buf.h

@@ -1027,6 +1027,14 @@ static inline unsigned int br_count(const struct buffer *r)
 		return r->data + r->size - r->head;
 }
 
+/* Returns true if a single buffer is assigned */
+static inline unsigned int br_single(const struct buffer *r)
+{
+	BUG_ON_HOT(r->area != BUF_RING.area);
+
+	return r->data == r->head;
+}
+
 /* Returns the index of the ring's head buffer */
 static inline unsigned int br_head_idx(const struct buffer *r)
 {