vxlan fdb replace an existing entry

Add support to replace an existing entry found in the
vxlan fdb database. The entry in question is identified
by its unicast mac address and the destination information
is changed. If the entry is not found, it is added in the
forwarding database. This is similar to changing an entry
in the neighbour table.

Multicast mac addresses can not be changed with the replace
option.

This is useful for virtual machine migration when the
destination of a target virtual machine changes. The replace
feature can be used instead of delete followed by add.

Resubmitted because net-next was closed last week.

Signed-off-by: Thomas Richter <tmricht@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>