net/mlx5: Fix steering memory leak

Flow steering priority and namespace are software only objects that
didn't have the proper destructors and were not freed during steering
cleanup.

Fix it by adding destructor functions for these objects.

Fixes: bd71b08e ("net/mlx5: Support multiple updates of steering rules in parallel")
Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>