net/mlx5e: Fix possible use-after-free deleting fdb rule

After neigh-update-add failure we are still with a slow path rule but
the driver always assume the rule is an fdb rule.
Fix neigh-update-del by checking slow path tc flag on the flow.
Also fix neigh-update-add for when neigh-update-del fails the same.

Fixes: 5dbe906f

 ("net/mlx5e: Use a slow path rule instead if vxlan neighbour isn't available")
Signed-off-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Paul Blakey <paulb@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>