Fix for btrfs check.

The Nagios plugin doesn't work on the device, it needs a mountpoint.

Ansible/roles/debian_server/tasks/btrfs.yml

 ---
+- name: Find a mountpoint for {{ device }}
+  # There can be multiple once with bind mounts or subvolumes, we only need
+  # one.
+  set_fact:
+    # yamllint disable rule:line-length
+    btrfs_mountpoint: |-
+      {{ (ansible_facts.mounts|selectattr("device", "equalto", device)|first)["mount"] }}
+    # yamllint enable rule:line-length
+
 - name: Install btrfs packages
   apt:
     cache_valid_time: 3600

Ansible/roles/debian_server/templates/btrfs_check.service.j2

@@ -6,4 +6,4 @@ After=network-online.target
 
 [Service]
 Type=exec
-ExecStart=mnpw --warn -- /usr/lib/nagios-btrfs/plugins/check_btrfs --mountpoint {{ device }}
+ExecStart=mnpw --warn -- /usr/lib/nagios-btrfs/plugins/check_btrfs --mountpoint {{ btrfs_mountpoint }}