Add the 'fdb_miss' key element to supported key blocks and make use of
it to match on layer 2 miss.

The key is only supported on Spectrum-{2,3,4}. An error is returned for
Spectrum-1 since the key element is not present in any of its key
blocks.
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>