Send neighbors into NUD_PROBE rather than NUD_DELAY

Now that device kernels have the required kernel patch [1] forcing
neighbors into NUD_PROBE guarantees that:

    (a) all neighbors will be probed, even currently reachable ones
    (b) no spurious neighbor failures will caused

[1] http://git.kernel.org/cgit/linux/kernel/git/davem/net-next.git/commit/?id=765c9c639fbb132af0cafc6e1da22fe6cea26bb8

Bug: 18581716
Change-Id: I51ba92a730ceb9a6c5193dfa6d9a0f1d10bbf16b