tmpdir-setup: Make umount faster

The umount of the job tmpdir often takes a lot of time (minutes!) when
the user write a lot of data to it, because there are many dirty pages
which are written to the disk before the umount completes.

This writing is obviously a waste of resources, because as soon as the
unmount is finished, the backup image for the filesystem is destroyed
anyway.

Unfortunatly, we currenty don't have a solid way to avoid the
unnecessary writeback.

What we can do is to purge the directory. Experiments show, that this is
indeed faster with both: few very big files or many small files. So do
that.

Hard-code the directory prefix into the rm command to decrease the risk
of rm removing wrong files in case future code changes are buggy and,
for example, missspell a shell variable.

Because the cleanup still needs time, do that in the background so that
mxqd can continue.

helper/tmpdir-setup

@@ -52,10 +52,12 @@ cmd_cleanup() {
     (( $# == 1 )) || usage
     MXQ_JOBID=$1
 
-    mountpoint=$mntdir/$MXQ_JOBID
-
-    umount $mountpoint
-    rmdir $mountpoint
+    (
+        shopt -s dotglob;
+        rm -rf /dev/shm/mxqd/mnt/job/$MXQ_JOBID/*
+        umount /dev/shm/mxqd/mnt/job/$MXQ_JOBID
+        rmdir /dev/shm/mxqd/mnt/job/$MXQ_JOBID
+    ) &
 }
 
 (( $# > 0 )) || usage