vxlan: Add vxlan_fdb_find_uc() for FDB querying

A switchdev-capable driver that is aware of VXLAN may need to query
VXLAN FDB. In the particular case of mlxsw, this functionality is
limited to querying UC FDBs. Those being easier to deal with than the
general case of RDST chain traversal, introduce an interface to query
specifically UC FDBs: vxlan_fdb_find_uc().

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