neigh: allow admin to set NUD_STALE

Admin should be able to set any state. Currently, this fails
when lladdr is not changed and state is changed from
NUD_CONNECTED to NUD_STALE:

ip neigh add 192.168.8.1 lladdr 00:11:22:33:44:55 nud perm dev wlan0
ip neigh show to 192.168.8.1
192.168.8.1 dev wlan0 lladdr 00:11:22:33:44:55 PERMANENT
ip neigh change 192.168.8.1 lladdr 00:11:22:33:44:55 nud stale dev wlan0
ip neigh show to 192.168.8.1
192.168.8.1 dev wlan0 lladdr 00:11:22:33:44:55 PERMANENT

Problem may be from 2.1.X days.
Signed-off-by: NJulian Anastasov <ja@ssi.bg>
Reviewed-by: NChunhui He <hchunhui@mail.ustc.edu.cn>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/neighbour.c

@@ -1148,7 +1148,8 @@ int neigh_update(struct neighbour *neigh, const u8 *lladdr, u8 new,
 			} else
 				goto out;
 		} else {
-			if (lladdr == neigh->ha && new == NUD_STALE)
+			if (lladdr == neigh->ha && new == NUD_STALE &&
+			    !(flags & NEIGH_UPDATE_F_ADMIN))
 				new = old;
 		}
 	}