[NETFILTER]: more verbose return codes from nf_{log,queue}

This adds EEXIST to distinguish between the following return values:
0: 	nobody was registered, registration successful
EEXIST:	the exact same handler was already registered, no registration
	required
EBUSY:	somebody else is registered, registration unsuccessful.

Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/netfilter/nf_log.c b/net/netfilter/nf_log.c
index 31a9d63..e104760 100644
--- a/net/netfilter/nf_log.c
+++ b/net/netfilter/nf_log.c
@@ -18,6 +18,8 @@
 static struct nf_logger *nf_logging[NPROTO]; /* = NULL */
 static DEFINE_SPINLOCK(nf_log_lock);
 
+/* return EBUSY if somebody else is registered, EEXIST if the same logger
+ * is registred, 0 on success. */
 int nf_log_register(int pf, struct nf_logger *logger)
 {
 	int ret = -EBUSY;
@@ -28,7 +30,9 @@
 	if (!nf_logging[pf]) {
 		rcu_assign_pointer(nf_logging[pf], logger);
 		ret = 0;
-	}
+	} else if (nf_logging[pf] == logger)
+		ret = -EEXIST;
+
 	spin_unlock(&nf_log_lock);
 	return ret;
 }