mlxsw: spectrum_router: Mark RIF index as taken before creation

The presence of an allocated RIF in mlxsw_sp->router->rifs[rif_index] marks
that rif_index as taken.
Set the marking of a taken RIF to happen before calling ops->create in
order to allow creation of a GRE underlay RIF, which may be allocated and
created as part of an overlay RIF creation.

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