Remove zram size limit and disable zswap when using zram

Two fixes that I tested and deployed to build.archlinux.org:

zram-generator: Uncap max size of the zram device

Set "max-zram-size = none" to disable this unwanted limitation which
defaulted to creating zram-based swap with a maximum size of 4096MiB.

Fixes: dc8fa2bdfc86 ("common: Replace deprecated systemd-swap[1] with zram-generator")

zram-generator: use systemd-zram-setup@zram0 handler

Restarting swap.target doesn't apply configuration changes; instead we
can restart systemd-zram-setup@zram0 which seems to do what we wanted.

Bonus commit so that zramctl will display useful statistics:

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

@@ -141,9 +141,18 @@
 - name: install zram-generator config for zram
   copy: src=zram-generator.conf dest=/etc/systemd/zram-generator.conf owner=root group=root mode=0644
   notify:
-    - restart swap.target
+    - 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