mlxsw: spectrum_router: Relax GRE decap matching check

GRE decap offload is configured when local routes prefix correspond to the
local address of one of the offloaded GRE tunnels. The matching check was
found to be too strict, such that for a flat GRE configuration, in which
the overlay and underlay traffic share the same non-default VRF, decap flow
was not offloaded.

Relax the check for decap flow offloading. A match occurs if the local
address of the tunnel matches the local route address while both share the
same VRF table.

Fixes: 4607f6d2

 ("mlxsw: spectrum_router: Support IPv4 underlay decap")
Signed-off-by: Nir Dotan <nird@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>