neigh: sysctl - simplify address calculation of gc_* variables

The code in neigh_sysctl_register() relies on a specific layout of
struct neigh_table, namely that the 'gc_*' variables are directly
following the 'parms' member in a specific order. The code, though,
expresses this in the most ugly way.

Get rid of the ugly casts and use the 'tbl' pointer to get a handle to
the table. This way we can refer to the 'gc_*' variables directly.

Similarly seen in the grsecurity patch, written by Brad Spengler.
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Cc: Brad Spengler <spender@grsecurity.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/neighbour.h

@@ -203,7 +203,6 @@ struct neigh_table {
 	void			(*proxy_redo)(struct sk_buff *skb);
 	char			*id;
 	struct neigh_parms	parms;
-	/* HACK. gc_* should follow parms without a gap! */
 	int			gc_interval;
 	int			gc_thresh1;
 	int			gc_thresh2;

net/core/neighbour.c

@@ -3059,11 +3059,12 @@ int neigh_sysctl_register(struct net_device *dev, struct neigh_parms *p,
 		memset(&t->neigh_vars[NEIGH_VAR_GC_INTERVAL], 0,
 		       sizeof(t->neigh_vars[NEIGH_VAR_GC_INTERVAL]));
 	} else {
+		struct neigh_table *tbl = p->tbl;
 		dev_name_source = "default";
-		t->neigh_vars[NEIGH_VAR_GC_INTERVAL].data = (int *)(p + 1);
-		t->neigh_vars[NEIGH_VAR_GC_THRESH1].data = (int *)(p + 1) + 1;
-		t->neigh_vars[NEIGH_VAR_GC_THRESH2].data = (int *)(p + 1) + 2;
-		t->neigh_vars[NEIGH_VAR_GC_THRESH3].data = (int *)(p + 1) + 3;
+		t->neigh_vars[NEIGH_VAR_GC_INTERVAL].data = &tbl->gc_interval;
+		t->neigh_vars[NEIGH_VAR_GC_THRESH1].data = &tbl->gc_thresh1;
+		t->neigh_vars[NEIGH_VAR_GC_THRESH2].data = &tbl->gc_thresh2;
+		t->neigh_vars[NEIGH_VAR_GC_THRESH3].data = &tbl->gc_thresh3;
 	}
 
 	if (handler) {