roles/borg-client: Fix backup script behavior for non-btrfs systems

roles/borg-client/templates/borg-backup.sh.j2

@@ -38,6 +38,8 @@ if is_btrfs "$src"; then
         btrfs subvolume snapshot -r "$vol" "$vol/$snapshotdir"
         mount -o bind "$vol/$snapshotdir" "$backup_mountdir/$vol"
     done
+else
+    backup_mountdir="$src"
 fi
 
 systemctl is-active postgresql && /usr/local/bin/backup-postgres.sh || true
@@ -61,10 +63,12 @@ borg create -v --stats -C lz4 \
     {{ backup_host }}:{{ backup_dir }}::$(date "+%Y%m%d-%H%M%S") "$backup_mountdir"
 borg prune -v {{ backup_host }}:{{ backup_dir }} --keep-daily=7 --keep-weekly=4 --keep-monthly=6
 
-umount -R "$backup_mountdir"
-for vol in $list_of_btrfs_submounts; do
-    delete_snapshot "$vol/$snapshotdir"
-done
-rmdir "$backup_mountdir"
+if is_btrfs "$src"; then
+    umount -R "$backup_mountdir"
+    for vol in $list_of_btrfs_submounts; do
+        delete_snapshot "$vol/$snapshotdir"
+    done
+    rmdir "$backup_mountdir"
+fi
 
 /usr/bin/vendor_perl/borg-restore.pl --update-cache