iproute2: fix the link group name getting error

iproute2: fix the link group name getting error

In the situation where more than one entry live in the same hash bucket,
loop to get the correct one.

Before:
$ cat /etc/iproute2/group
0	default
256     test

$ sudo ip link set group test dummy1

$ ip link show type dummy
11: dummy0: <BROADCAST,NOARP> mtu 1500 qdisc noop state DOWN mode DEFAULT group 0 qlen 1000
    link/ether 4e:3b:d3:6c:f0:e6 brd ff:ff:ff:ff:ff:ff
12: dummy1: <BROADCAST,NOARP> mtu 1500 qdisc noop state DOWN mode DEFAULT group test qlen 1000
    link/ether d6:9c:a4:1f:e7:e5 brd ff:ff:ff:ff:ff:ff

After:
$ ip link show type dummy
11: dummy0: <BROADCAST,NOARP> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ether 4e:3b:d3:6c:f0:e6 brd ff:ff:ff:ff:ff:ff
12: dummy1: <BROADCAST,NOARP> mtu 1500 qdisc noop state DOWN mode DEFAULT group test qlen 1000
    link/ether d6:9c:a4:1f:e7:e5 brd ff:ff:ff:ff:ff:ff

Signed-off-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>