Avoid 100MB reserved space in private ext4 partition

The ext4 reserved space is necessary for root partitions, but in the
private.img data partition, it is wasted space (accessible only to root
processes), which means losing 100 MB of the default 2GB.

From mkfs.ext4 man page:
"-m reserved-blocks-percentage Specify the percentage of the filesystem
blocks reserved for the super-user." ... "The default percentage is 5%."

templates.spec

@@ -83,7 +83,7 @@ if [ "$1" = 1 ] ; then
     # installing for the first time
     echo "--> Creating private.img..."
     truncate -s 2G %{dest_dir}/private.img
-    mkfs.ext4 -q -F %{dest_dir}/private.img
+    mkfs.ext4 -m 0 -q -F %{dest_dir}/private.img
     chown root.qubes %{dest_dir}/private.img
     chmod 0660 %{dest_dir}/private.img
 fi