services: file-system: Change directory to root before unmounting.

* gnu/services/base.scm (file-system-service)[stop]: Add 'chdir' call.

gnu/services/base.scm

@@ -137,6 +137,10 @@ (define* (file-system-service device target type
       (stop #~(lambda args
                 ;; Normally there are no processes left at this point, so
                 ;; TARGET can be safely unmounted.
+
+                ;; Make sure PID 1 doesn't keep TARGET busy.
+                (chdir "/")
+
                 (umount #$target)
                 #f))))))