netdevsim: correctly check return value of debugfs_create_dir

- Checking return value with IS_ERROR_OR_NULL
- Added error handling where it was not handled

Signed-off-by: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>