net/mlx5: E-Switch, Add default drop rule for unmatched packets

The ft_offloads table serves to steer packets, which are from the
eswitch, to the representor associated with the packets' source vport.

Previously, if a packet's source vport or metadata was not associated
with any representor, it was forwarded to the uplink representor. The
representor got packets it shouldn't have as they weren't coming from
the uplink vport.

One such effect of this breakage can be observed if the uplink
representor is attached to a bridge, where such illegal packets will
be broadcast to the remaining ports, flooding the switch with illegal
packets. In the case where IB loopback (e.g, SNAP) is enabled, all
transmitted packets would be looped back, and received by the uplink
representor, and result in an infinite feedback loop.

Therefore, block this hole by adding a default drop rule to the
ft_offloads table, so that all unmatched packets with no associated
representor are dropped.

Signed-off-by: Jianbo Liu <jianbol@nvidia.com>
Reviewed-by: Gavi Teitz <gavi@nvidia.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>