roles/minimum_memory: Correct calculation errors.

When it already exists, the swap file managed by the role must be
excluded from the sum of memory and swap used to calculate the size of
said file.

The size of the swap file's header ought to be included in the
calculation; otherwise, the file will be slightly undersized, and the
role will re-create it at every invocation.

roles/minimum_memory/tasks/main.yml

@@ -16,10 +16,15 @@
       command: swapoff -- {{ swapfile_name | quote }}
       when: not r.rc
 
+    - name: "Update fact: ansible_swaptotal_mb."
+      setup:
+        filter: ansible_swaptotal_mb
+
     - name: Create swapfile.
-      command: dd if=/dev/zero of={{ swapfile_name | quote }} bs=1024k
-               count={{ minimum_memory_mb - ansible_memtotal_mb -
-                        ansible_swaptotal_mb }}
+      # The swapfile's header takes up 4 KiB.
+      command: dd if=/dev/zero of={{ swapfile_name | quote }} bs=1024
+               count={{ (minimum_memory_mb - ansible_memtotal_mb -
+                        ansible_swaptotal_mb) * 1024 + 4 }}
 
     - name: Set swapfile permissions.
       file:
@@ -39,4 +44,12 @@
 
     - name: Enable swapping for file.
       command: swapon -- {{ swapfile_name | quote }}
+
+    - name: "Update fact: ansible_swaptotal_mb."
+      setup:
+        filter: ansible_swaptotal_mb
+
+    # Nota bene: No tasks belong below this point in the block. The "when"
+    # clause is re-evaluated before each task, and the setup call above this
+    # will make it evaluate to false.
   when: ansible_memtotal_mb + ansible_swaptotal_mb < minimum_memory_mb