discover/udev: fix double free on udev_init failures

If the udev monitor or enumerate functions fail, we'll call the
udev_unref and udev_monitor_unref functions twice: once in the cleanup
path and once in the talloc destructor.

This change moves all cleanup to the talloc destructor, so we only do
the unrefs once.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>