mlxsw: Add support for egress FID classification after decapsulation

As preparation for unified bridge model, add support for VNI->FID mapping
via SVFA register.

When performing VXLAN encapsulation, the VXLAN header needs to contain a
VNI. This VNI is derived from the FID classification performed on
ingress, through which the ingress RIF is also determined.

Similarly, when performing VXLAN decapsulation, the FID of the packet
needs to be determined. This FID is derived from VNI classification
performed during decapsulation.

In the old model, both entries (i.e., FID->VNI and VNI->FID) were
configured via SFMR.vni.

In the new model, where ingress is separated from egress, ingress
configuration (VNI->FID) is performed via SVFA, while SFMR only
configures egress (FID->VNI).

Add 'vni' field to SVFA, add new mapping table - VNI to FID, add new
pack() function for VNI mapping and edit the comment in SFMR.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>