net/mlx5: Annotate mutex destroy for root ns

Invoke mutex_destroy() to catch any errors.

Fixes: 2cc43b49 ("net/mlx5_core: Managing root flow table")
Signed-off-by: Roi Dayan <roid@mellanox.com>
Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>