ip: fix exit codes

ip: fix exit codes

Alternative fix to problem reported by: Bin Li
The issue is came from https://bugzilla.novell.com/show_bug.cgi?id=681952.

In any previous version (since suse ... 10.0?), ip addr add always returned
the error code 2 in case the ip address is already set on the interface:

    inet 172.16.2.3/24 brd 172.16.2.255 scope global bond0
RTNETLINK answers: File exists
2

On 11.4, it returns the exit code 254:

    inet 172.16.1.1/24 brd 172.16.1.255 scope global eth0
RTNETLINK answers: File exists
254

This of course causes ifup to return an error in this quite common case..