nbd/nbdroot: use udevsettle to wait for /dev/nbd0

nbdroot is now in the initqueue, so we can use udevsettle

modules.d/95nbd/nbdroot

@@ -86,7 +86,11 @@ fsopts=${fsopts+$fsopts,}${nbdrw}
 i=0
 while [ ! -b /dev/nbd0 ]; do
     [ $i -ge 20 ] && exit 1
-    sleep 0.1
+    if [ $UDEVVERSION -ge 143 ]; then
+        udevadm settle --exit-if-exists=/dev/nbd0
+    else
+        sleep 0.1
+    fi
     i=$(( $i + 1))
 done