net/mlx5: fix empty flow error structure

net/mlx5: fix empty flow error structure

The output flow error parameter is used to indicate the detailed
reason of the failure when calling a rte_flow_* interface. Even
though sometimes the application will not check it or use it, the PMD
must fill it in the failure branch before returning. Or else, some
dirty value in the stack, heap will be accessed as a pointer and then
cause a crash.
In this case, when a port is stopped, it is not allowed to insert a
flow from application. The detailed error information should be
filled. If the application needs to check the detailed error reason,
it will get the information but not result in any crash.

Fixes: 40b9e7f65fe1 ("net/mlx5: check device status before creating flow")

Signed-off-by: Bing Zhao <bingz@mellanox.com>
Acked-by: Ori Kam <orika@mellanox.com>