MINOR: ring: add ring_data() to report the amount of data in a ring

This will be used as an accessor for the few functions that need this
outside of ring.c.

include/haproxy/ring.h

@@ -45,6 +45,12 @@ size_t ring_max_payload(const struct ring *ring);
 int ring_dispatch_messages(struct ring *ring, void *ctx, size_t *ofs_ptr, size_t *last_ofs_ptr, uint flags,
 			   ssize_t (*msg_handler)(void *ctx, const struct buffer *buf, size_t ofs, size_t len));
 
+/* returns the number of bytes in the ring */
+static inline size_t ring_data(const struct ring *ring)
+{
+	return b_data(&ring->buf);
+}
+
 #endif /* _HAPROXY_RING_H */
 
 /*