neigh: optimize neigh_parms_release()

In neigh_parms_release() we loop over all entries to find the entry given in
argument and being able to remove it from the list. By using a double linked
list, we can avoid this loop.

Here are some numbers with 30 000 dummy interfaces configured:

Before the patch:
$ time rmmod dummy
real	2m0.118s
user	0m0.000s
sys	1m50.048s

After the patch:
$ time rmmod dummy
real	1m9.970s
user	0m0.000s
sys	0m47.976s

Suggested-by: Thierry Herbelot <thierry.herbelot@6wind.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/neighbour.h b/include/net/neighbour.h
index f60558d..dedfb18 100644
--- a/include/net/neighbour.h
+++ b/include/net/neighbour.h
@@ -69,7 +69,7 @@
 	struct net *net;
 #endif
 	struct net_device *dev;
-	struct neigh_parms *next;
+	struct list_head list;
 	int	(*neigh_setup)(struct neighbour *);
 	void	(*neigh_cleanup)(struct neighbour *);
 	struct neigh_table *tbl;
@@ -203,6 +203,7 @@
 	void			(*proxy_redo)(struct sk_buff *skb);
 	char			*id;
 	struct neigh_parms	parms;
+	struct list_head	parms_list;
 	int			gc_interval;
 	int			gc_thresh1;
 	int			gc_thresh2;