Instead of watching /dev, use a separate watch for each device.
This is supposed to achieve the same result as the now reverted
88f0dbd6fc ("v4l2: don't set inotify on /dev"):
Doing inotify on /dev is not a good idea because we will be woken up by
a lot of unrelated events.
There is a report of a performance regression on some IO benchmark
because of lock contention within the fsnotify subsystem due to this.
Instead, just watch for attribute changes on the /dev/videoX files
directly. We are only interested in attribute changes, udev should
notify us when the file is added or removed.
Fixes#3439
3bbccccd05