net/mlx5: Avoid forwarding to other eswitch uplink

Do not allow forwarding of encapsulated traffic received from one eswtich's
uplink to another eswtich's uplink.

Signed-off-by: Eli Cohen <eli@mellanox.com>
Reviewed-by: Oz Shlomo <ozsh@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>