neigh: Better handling of transition to NUD_PROBE state

[1] When entering NUD_PROBE state via neigh_update(), perhaps received
    from userspace, correctly (re)initialize the probes count to zero.

    This is useful for forcing revalidation of a neighbor (for example
    if the host is attempting to do DNA [IPv4 4436, IPv6 6059]).

[2] Notify listeners when a neighbor goes into NUD_PROBE state.

    By sending notifications on entry to NUD_PROBE state listeners get
    more timely warnings of imminent connectivity issues.

    The current notifications on entry to NUD_STALE have somewhat
    limited usefulness: NUD_STALE is a perfectly normal state, as is
    NUD_DELAY, whereas notifications on entry to NUD_FAILURE come after
    a neighbor reachability problem has been confirmed (typically after
    three probes).

Change-Id: Idfbff01774453b0930da48edaf0dfb9da701387a
Signed-off-by: Erik Kline <ek@google.com>
Acked-By: Lorenzo Colitti <lorenzo@google.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Git-commit: 765c9c639fbb132af0cafc6e1da22fe6cea26bb8
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Acked-by: Nagarjuna Chaganti <nchagant@qti.qualcomm.com>
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>