tests: simplify parallel_cp.sh

Get rid of one extra directory level. Hopefully makes it
easier to follow the debug logs.

tests/stress_tests/parallel_cp.sh

@@ -1,7 +1,16 @@
 #!/bin/bash
 #
 # Simplified xfstests generic/273
-# See https://github.com/rfjakob/gocryptfs/issues/322
+#
+# Fails with
+#
+#   cp: cannot create regular file 'sub_49/file_773': No such file or directory
+#
+# If you cannot reproduce, try running this in the background:
+#
+#   while sleep 0.1 ; do echo 3 > /proc/sys/vm/drop_caches ; done"
+#
+# See https://github.com/rfjakob/gocryptfs/issues/322 for details.
 
 echo "deleting old files"
 rm -Rf origin sub_*
@@ -12,11 +21,13 @@ mkdir -p origin
 for i in $(seq 1 778) ; do
 	dd if=/dev/zero of=origin/file_$i bs=8192 count=1 status=none
 done
+# Perform the shell expansion only once and store the list
+ORIGIN_FILES=origin/*
 
 echo -n "cp starting: "
 for i in $(seq 1 100) ; do
 	echo -n "$i "
-	(mkdir sub_$i && cp -r origin sub_$i ; echo -n "$i ") &
+	(mkdir sub_$i && cp $ORIGIN_FILES sub_$i ; echo -n "$i ") &
 done
 
 echo