vxlan: Move up vxlan_fdb_free(), vxlan_fdb_destroy()

These functions will be needed for rollbacks of vetoed FDB entries. Move
them up so that they are visible at their intended point of use.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>