net/mlx5: Simplify a test

'create_root_ns()' does not return an error pointer, so the test can be
simplified to be more consistent.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>