storage: Force sync after preparing volatile.img

This makes possible to release d-m partitions devices sooner (so avoid
race with kpartx).

linux/aux-tools/prepare-volatile-img.sh

@@ -31,6 +31,7 @@ kpartx -s -a "$FILENAME"
 loopdev=`losetup -j "$FILENAME"|tail -n 1 |cut -d: -f1`
 looppart=`echo $loopdev|sed 's:dev:dev/mapper:'`
 mkswap -f ${looppart}p1 > /dev/null
+sync
 kpartx -s -d ${loopdev}
 losetup -d ${loopdev} || :
 chown --reference `dirname "$FILENAME"` "$FILENAME"