interface: Introduce an access method for the time stamping label.

Many of the users only require a read only reference to the time
stamping label of the interface.  This patch adds an appropriate
method.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>

interface.c

@@ -6,6 +6,11 @@
  */
 #include "interface.h"
 
+const char *interface_label(struct interface *iface)
+{
+	return iface->ts_label;
+}
+
 const char *interface_name(struct interface *iface)
 {
 	return iface->name;

interface.h

@@ -24,6 +24,15 @@ struct interface {
 	struct sk_ts_info ts_info;
 };
 
+/**
+ * Obtain the time stamping label of a network interface.  This can be
+ * different from the name of the interface when bonding is in effect.
+ *
+ * @param iface  The interface of interest.
+ * @return       The time stamping device name of the network interface.
+ */
+const char *interface_label(struct interface *iface);
+
 /**
  * Obtains the name of a network interface.
  * @param iface  The interface of interest.