Fix crating multiple loopback devices

This patch fixes issue with duplicate task IDs when multiple
loopback devices should be added, and they stored in the same
directory.

Change-Id: Iae0027b6ee187f5366967e667380dfb3eae232fc
Related-Prod: PROD-17620

linux/storage/loopback.sls

 
 {%- if loopback.get('enabled', True) %}
 
-{{ salt['file.dirname'](loopback.file) }}:
+{{ loopback.file }}_directory:
   file.directory:
+  - name: {{ salt['file.dirname'](loopback.file) }}
   - makedirs: true
   - require_in:
     - file: {{ loopback.file }}