netdevsim: make debug dirs' dentries static

The root directories of netdevsim should only be used by the core
to create per-device subdirectories, so limit their visibility to
the core file.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>