loop-util: read kernel's uevent seqnum right before attaching a loopback device

loop-util: read kernel's uevent seqnum right before attaching a loopback device

Later, this will allow us to ignore uevents from earlier attachments a
bit better, as we can compare uevent seqnums with this boundary. It's
not a full fix for the race though, since we cannot atomically determine
the uevent and attach the device, but it at least shortens the window a
bit.