Add support for the bridge fdb replace command to replace an
existing entry in the vxlan device driver forwarding data base.
The entry is identified with its unicast mac address and its
corresponding remote destination information is updated.

This is useful for virtual machine migration and replaces the
bridge fdb del and bridge fdb add commands.

It follows the same interface as ip neigh replace commands.
Signed-off-by: Thomas Richter <tmricht@linux.vnet.ibm.com>