Move ARC size to defaults so it can be easily changed

ansible/roles/zfs/defaults/main.yml

@@ -0,0 +1,2 @@
+# Cap ARC size to 50% RAM
+zfs_arc_size: "{{ (ansible_memtotal_mb * 1024 * 1024) * 0.5 }}"

ansible/roles/zfs/tasks/main.yml

@@ -1,7 +1,3 @@
-- name: Calculate ZFS ARC size
-  set_fact:
-    zfs_arc_size: "{{ (ansible_memtotal_mb * 1024 * 1024) * 0.5 }}"
-
 - name: Enable zfs module
   modprobe:
     name: zfs