MINOR: buffer: Add b_set_data().

Add a new function that lets you set the amount of input in a buffer.
For now it extends/truncates b->i except if the total length is
below b->o in which case it clears i and adjusts o.

include/common/buf.h

@@ -311,6 +311,17 @@ static inline void bo_add(struct buffer *b, size_t count)
 	b->o += count;
 }
 
+/* b_set_data() : sets the buffer's length */
+static inline void b_set_data(struct buffer *b, size_t len)
+{
+	if (len >= b->o)
+		b->i = len - b->o;
+	else {
+		b->o = len;
+		b->i = 0;
+	}
+}
+
 
 #endif /* _COMMON_BUF_H */