fix #1691: replace udev check

fix #1691: replace udev check

`zfs create` add the creation job in a worker queue,
which should normally execute instantly. But there are circumstances
where the job will take a while to get processed.
If this is the case udev settle will see no dev in the queue and the program
will continue without an allocated dev.

The busy waiting is not best practice but the only way to be sure,
that the block device exists.