tmpdir-setup: Don't umount in background

The old cleanup code runs in the background, which is okay for normal
operations. However when mxqd restarts and finds jobs, which have
finished in the meantime, it calls the tmpdir cleanup code for theses jobs.

Later during recovery, it scans the spool area for other remaining
tmpdir mounts and calls the cleanup code for these. This races with the
oridnary cleanup when it is asynchronous.

This bug exists in 0.30.7 and has nothing to do with the current
changes. It is only triggered when a restarting mxqd finds completed
jobs. There is no harm done, the jobs complete normally, the tmpdir is
cleand up. The second attempted cleanup just produces some error
messages in the log. Without any other side effects.

As the "old style" tmpdir setup is going away anyway, don't invent
something complicated here and just do the cleanup synchronously.

helper/tmpdir-setup

@@ -61,12 +61,10 @@ cmd_cleanup() {
     # Support rolling upgrade: Use old code if loop device is directly mounted without a dm device in between.
     #
     if [[ $(findmnt --output SOURCE --noheadings /dev/shm/mxqd/mnt/job/$MXQ_JOBID) =~ ^/dev/loop ]]; then
-        (
-            shopt -s dotglob
-            rm -rf /dev/shm/mxqd/mnt/job/$MXQ_JOBID/*
-            umount /dev/shm/mxqd/mnt/job/$MXQ_JOBID
-            rmdir /dev/shm/mxqd/mnt/job/$MXQ_JOBID
-        ) &
+        shopt -s dotglob
+        rm -rf /dev/shm/mxqd/mnt/job/$MXQ_JOBID/*
+        umount /dev/shm/mxqd/mnt/job/$MXQ_JOBID
+        rmdir /dev/shm/mxqd/mnt/job/$MXQ_JOBID
         exit
     fi