spa v4l2 fails on login

Early during login, devices tagged with uaccess are not always yet available. There is a work-around for alsa devices using inotify.

For v4l2, there are also calls to inotify_add_watch. But to the devices directly, that is /dev/video0, /dev/video1. Unfortunately, it is not possible to add a watch on a file that is not readable. It returns an error.

When logging in with a webcam connected, wireplumber has this failures in the logs: inotify_add_watch() failed: Permission denied. This is from the spa v4l2 plugin.

Instead of calling inotify_add_watch for every device. v4l2 should call it just for /dev and filter what device in the callback.