neigh: use neigh_parms_net() to get struct neigh_parms->net pointer

This fixes compile error when CONFIG_NET_NS is not set.

Introduced by:
commit 1d4c8c29
    "neigh: restore old behaviour of default parms values"
Signed-off-by: NJiri Pirko <jiri@resnulli.us>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/neighbour.c

@@ -2850,7 +2850,7 @@ static void neigh_proc_update(struct ctl_table *ctl, int write)
 {
 	struct net_device *dev = ctl->extra1;
 	struct neigh_parms *p = ctl->extra2;
-	struct net *net = p->net;
+	struct net *net = neigh_parms_net(p);
 	int index = (int *) ctl->data - p->data;
 
 	if (!write)