rtnetlink: propagate error from dev_change_flags in do_setlink()

Unlike ifconfig, iproute doesn't report an error when setting
an interface up fails:

(example: put wireless network mac80211 interface into repeater mode
with iwconfig but do not set a peer MAC address, it should fail with
-ENOLINK)

without patch:
# ip link set wlan0 up ; echo $?
0
# 

with patch:
# ip link set wlan0 up ; echo $?
RTNETLINK answers: Link has been severed
2
# 

Propagate the return value from dev_change_flags() to fix this.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Tested-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -878,7 +878,9 @@ static int do_setlink(struct net_device *dev, struct ifinfomsg *ifm,
 		if (ifm->ifi_change)
 			flags = (flags & ifm->ifi_change) |
 				(dev->flags & ~ifm->ifi_change);
-		dev_change_flags(dev, flags);
+		err = dev_change_flags(dev, flags);
+		if (err < 0)
+			goto errout;
 	}
 
 	if (tb[IFLA_TXQLEN])