net: devlink: move net check into devlinks_xa_for_each_registered_get()

Benefit from having devlinks iterator helper
devlinks_xa_for_each_registered_get() and move the net pointer
check inside.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>