IPoIB: Fix network interface "RUNNING" status

With the current IPoIB driver, the status of network interfaces stays
"RUNNING" even if the link goes down (for example because a cable is
unplugged).  Fix this by flushing the IPoIB interface when the link
goes down.

Signed-off-by: Leonid Arsh <leonida@voltaire.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>