init-storage: Use a static path as mount point

There's no particular reason why the directory used as the temporary
mount point for the data volume needs to be random.  Using a static
name, on the other hand, makes it easier for the SELinux policy to
apply the correct type transition and ensure the directory is labelled
correctly.

overlay/usr/libexec/init-storage

@@ -15,8 +15,8 @@ copy_var() {
     dev="$1"
 
     echo 'Copying /var contents to data volume'
-    mount -o subvol=var "${dev}" "${tmpdir}"
-    cp -auv /var/. "${tmpdir}"
+    mount -o subvol=var "${dev}" "${tmpdir}" || exit
+    cp -auv /var/. "${tmpdir}" || exit
     umount "${tmpdir}"
 }
 
@@ -46,7 +46,8 @@ else
 fi
 
 trap cleanup INT TERM QUIT EXIT
-tmpdir=$(mktemp -d -p /run storinit.XXXXXX)
+tmpdir=/run/storinit
+mkdir -p "${tmpdir}"
 
 if ! has_fs "${datapart}"; then
     format_dev "${datapart}"