net/mlx5: fix flow priorities probing error path

net/mlx5: fix flow priorities probing error path

The mlx5 PMD probes the Verbs flow priorities supported with
ibv_create_flow() function. If rdma-core or kernel fails for
some reason, the returned error causes the drop queue is not
destroyed, and pd is locked by not freed resource.

Also the mlx5_flow_discover_priorities() returned negative value
as error, and this code was reported "as is", without sign
changing (eventually causing assert(err > 0)).

Fixes: 2815702baea7 ("net/mlx5: replace verbs priorities by flow")
Cc: stable@dpdk.org
Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Acked-by: Shahaf Shuler <shahafs@mellanox.com>