bridge: allow creating/deleting fdb entries via netlink

Use RTM_NEWNEIGH and RTM_DELNEIGH to allow updating of entries
in bridge forwarding table. This allows manipulating static entries
which is not possible with existing tools.

Example (using bridge extensions to iproute2)
   # br fdb add 00:02:03:04:05:06 dev eth0

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