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: Julian Anastasov <ja@ssi.bg>
Reviewed-by: Chunhui He <hchunhui@mail.ustc.edu.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>