drop_monitor: Add alert mode operations

The next patch is going to add another alert mode in which the dropped
packet is notified to user space, instead of only a summary of recent
drops.

Abstract the differences between the modes by adding alert mode
operations. The operations are selected based on the currently
configured mode and associated with the probes and the work item just
before tracing starts.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/net_dropmon.h

@@ -62,4 +62,13 @@ enum {
  * Our group identifiers
  */
 #define NET_DM_GRP_ALERT 1
+
+/**
+ * enum net_dm_alert_mode - Alert mode.
+ * @NET_DM_ALERT_MODE_SUMMARY: A summary of recent drops is sent to user space.
+ */
+enum net_dm_alert_mode {
+	NET_DM_ALERT_MODE_SUMMARY,
+};
+
 #endif