mlxsw: spectrum_router: Don't create FIB node during lookup

When looking up a FIB entry we shouldn't create the FIB node where it's
supposed to be linked in case the node doesn't already exist.

Instead, lookup the node and fail if it doesn't exist.

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