mlxsw: spectrum_router: Use existing decap route

The local route that points at IPIP's underlay device (decap route) can
be present long before the GRE device. Thus when an encap route is
added, it's necessary to look inside the underlay FIB if the decap route
is already present. If so, the current trap offload needs to be
withdrawn and replaced with a decap offload.

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