drop_monitor: allow more events per second

It seems there is a logic error in trace_drop_common(), since we store
only 64 drops, even if they are from same location.

This fix is a one liner, but we probably need more work to avoid useless
atomic dec/inc

Now I can watch 1 Mpps drops through dropwatch...
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/drop_monitor.c

@@ -150,6 +150,7 @@ static void trace_drop_common(struct sk_buff *skb, void *location)
 	for (i = 0; i < msg->entries; i++) {
 		if (!memcmp(&location, msg->points[i].pc, sizeof(void *))) {
 			msg->points[i].count++;
+			atomic_inc(&data->dm_hit_count);
 			goto out;
 		}
 	}