net/mlx5: fix use of local array for global error

net/mlx5: fix use of local array for global error

Recent patch uses a local string array as input for function
rte_flow_error_set().
This stack memory may be later used by other code sections,
overwriting the desired error string.

This patch implements an error string for the specific case
requested, of ICMP item not supported in Verbs flow engine.

Fixes: d51475d1bfa5 ("net/mlx5: support item type error message in flow Verbs")

Signed-off-by: Dekel Peled <dekelp@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>