nbd/001: wait for the device node to show up before running parted

The parted call can happen before the device is settled and thus fail.
Currently this happens very rarely for me (about 1 in 500 runs), but
a pending change to freeze the queues for updating the limits will make
it much more likely to hit.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>

tests/nbd/001

@@ -18,6 +18,8 @@ test() {
 	echo "Running ${TEST_NAME}"
 	_start_nbd_server
 	nbd-client -L -N export localhost /dev/nbd0 >> "$FULL" 2>&1
+	udevadm settle
+
 	parted -s /dev/nbd0 print 2>> "$FULL" | grep 'Disk /dev/nbd0'
 	lsblk --raw --noheadings /dev/nbd0