cxgb3: Handle ARP completions that mark neighbors stale.

When ARP completes due to a request rather than a reply the neighbor is
marked NUD_STALE instead of reachable (see arp_process()).  The handler
for the resulting netevent needs to check also for NUD_STALE.

Failure to use the arp entry can cause RDMA connection failures.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Acked-by: Divy Le Ray <divy@chelsio.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>