selftests: mlxsw: Add ingress RIF configuration test for VXLAN

Before layer 2 forwarding, the device classifies an incoming packet to a
FID. After classification, the FID is known, but also all the attributes of
the FID, such as the router interface (RIF) via which a packet that needs
to be routed will ingress the router block.

For VXLAN decapsulation, the FID classification is done according to the
VNI. When a RIF is added on top of a FID, the existing VNI->FID mapping
should be updated by the software with the new RIF. In addition, when a new
mapping is added for FID which already has a RIF, the correct RIF should
be used for it.

Add a test to verify that packets can be routed after decapsulation which
is done after VNI->FID classification, regardless of the order of the
configuration.

 # ./ingress_rif_conf_vxlan.sh
 TEST: Add RIF for existing VNI->FID mapping                         [ OK ]
 TEST: Add VNI->FID mapping for FID with a RIF                       [ OK ]

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>