netdevsim: don't leak devlink resources

Devlink resources registered with devlink_resource_register() have
to be unregistered.

Fixes: 37923ed6 ("netdevsim: Add simple FIB resource controller via devlink")
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>