fix from David Daney, on busybox mailing list:

fix from David Daney, on busybox mailing list:
 > The problem is that if the zcip protocol times out at about the same
 > time another ARP packet is being received, the newly calculated timeout
 > may be set to wait forever.  This prevents the protocol from progressing
 > through its various states.
 >
 > The Fix is to set the timeout to zero if it used to be a positive
 > number, but it has already expired.  This causes the next protocol state
 > to be entered immediately instead of never.
 >
 > If OK please commit.
 >
 > Also note that if you may have to apply the patch on this page:
 >
 > http://www.science.uva.nl/research/air/wiki/LinkLocalARPMeasurements
 >
 > To the linux kernel to get proper zcip behavior.
 >