Create subvolumes in init-storage

It turns out that we cannot use `systemd-tmpfiles` to create our Btrfs
subvolumes.  Since the directories we are interested in, specifically
`/var/log` and `/var/tmp` already exist in the rootfs image and are
therefore copied into the mutable filesystem, `systemd-tmpfiles` ignores
them.

To avoid having to explicitly specify the SELinux context for each
subvolume created on the persistent filesystem, `init-storage` now
executes `setfiles` to set the appropriate labels.

overlay/usr/libexec/init-storage

@@ -1,6 +1,13 @@
 #!/bin/sh
 # vim: set sw=4 ts=4 sts=4 et :
 
+SUBVOLUMES='
+    /var
+    /var/log
+    /var/tmp
+    /etc
+'
+
 cleanup() {
     if [ -n "${tmpdir}" ] && [ "${tmpdir}" != / ]; then
         if mountpoint -q "${tmpdir}"; then
@@ -26,10 +33,11 @@ format_dev() {
     mkfs.btrfs "${dev}" || exit
 
     mount "${dev}" "${tmpdir}" || exit
-    btrfs subvolume create "${tmpdir}"/var || exit
-    chcon -t var_t "${tmpdir}"/var || exit
-    btrfs subvolume create "${tmpdir}"/etc || exit
-    chcon -t etc_t "${tmpdir}"/etc || exit
+    for vol in ${SUBVOLUMES}; do
+        mkdir -p "${tmpdir}${vol%/*}" || exit
+        btrfs subvolume create "${tmpdir}${vol}" || exit
+    done
+    relabel_all
     umount "${dev}" || exit
 }
 
@@ -39,6 +47,18 @@ has_fs() {
     [ -n "${fstype}" ]
 }
 
+relabel_all() {
+    selinuxtype=$(. /etc/selinux/config && echo ${SELINUXTYPE})
+    find "${tmpdir}" | \
+    setfiles \
+        -v \
+        -F \
+        -m \
+        -r "${tmpdir}" \
+        -s \
+        /etc/selinux/${selinuxtype}/contexts/files/file_contexts
+}
+
 setup_etc() {
     dev="$1"