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: NPatrick McHardy <kaber@trash.net>
Tested-by: NJohannes Berg <johannes@sipsolutions.net>
Signed-off-by: NJohannes Berg <johannes@sipsolutions.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>