zram: Disable zswap to prevent conflict with zram

When both zswap and zram are active, zswap sits in front of zram and
treats it as a backing store. We just want to use zram and not zswap
disguising itself as such; disable the latter so we can enjoy useful
zramctl statistics.

Implemented as tmpfiles.d/zram.conf which disables zswap at runtime.

roles/common/tasks/main.yml

@@ -144,6 +144,15 @@
     - restart systemd-zram-setup@zram0
   when: enable_zram_swap
 
+- name: disable zswap to prevent conflict with zram
+  copy: content="w- /sys/module/zswap/parameters/enabled - - - - N" dest=/etc/tmpfiles.d/zram.conf
+  register: zramtmpfiles
+  when: enable_zram_swap
+
+- name: use tmpfiles.d/zram.conf
+  command: systemd-tmpfiles --create
+  when: zramtmpfiles.changed
+
 - name: start systemd-oomd
   service: name=systemd-oomd state=started enabled=yes