mlxsw: spectrum_router: Relax FIB rule validation

Currently, mlxsw does not support policy-based routing (PBR) and
therefore forbids the installation of non-default FIB rules except for
the l3mdev rule which is used for VRFs.

Relax the check to allow the installation of FIB rules that would never
match packets received by the device. Specifically, if the iif is that
of the loopback netdev. This is useful for users that need to redirect
locally generated packets based on FIB rules.

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