zdtm: suppress error messages if a cgroup holder exists

mkdir: cannot create directory ‘zdtm.mpAWGL/holder’: File exists
Reported-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/zdtm_mount_cgroups

@@ -10,7 +10,7 @@ tdir=`mktemp -d zdtm.XXXXXX`
 for i in "zdtmtst" "zdtmtst.defaultroot"; do
 	mount -t cgroup -o none,name=$i zdtm $tdir &&
 	# a fake group prevents destroying of a controller
-	mkdir $tdir/holder &&
+	mkdir -p $tdir/holder &&
 	umount $tdir || exit 1
 done
 rmdir $tdir