mlxsw: spectrum: Add trap for decapsulated ARP packets

After a packet was decapsulated it is classified to the relevant FID
based on its VNI and undergoes L2 forwarding.

Unlike regular (non-encapsulated) ARP packets, Spectrum does not trap
decapsulated ARP packets during L2 forwarding and instead can only trap
such packets in the underlay router during decapsulation.

Add this missing packet trap, which is required for VXLAN routing when
the MAC of the target host is not known.

Fixes: b02597d5 ("mlxsw: spectrum: Add NVE packet traps")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>