bridge: add NTF_USE support

More changes to the recent code to support control of forwarding
database via netlink.
   * Support NTF_USE like neighbour table
   * Validate state bits from application
   * Only send notifications (and change bits) if new entry is
     different.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>