Use v4l2_device_set_name to set name for added v4l2 loopback devices

[wt]

This should be a pretty easy one. The module source currently does a manual snprintf to set the device's name upon creation of a new loopback device. Instead, those calls should use v4l2_device_set_name.

Possible affected lines @ https://github.com/umlaeute/v4l2loopback/blob/main/v4l2loopback.c#L2418-L2419 and https://github.com/umlaeute/v4l2loopback/blob/main/v4l2loopback.c#L2440.

FWIW, I am filing this so that I don't forget to take a closer look.