Reencryption happens before resizing, so it only takes
about two minutes on the phone.

In order to use it, initial LUKS dump needs to be stored
on the rootfs:

    mkdir -p /var/lib/reencrypt
    LUKSDEV=$(cryptsetup status crypt_root | awk -F: '$1~/device/ {gsub(/^[ \t]+/, "", $2); print $2}')
    cryptsetup luksDump $LUKSDEV > /var/lib/reencrypt/dump