case-anon-cow-rand-mt: remove the /N for thread case

As with commit 37f440c6eb71("case-anon-cow-seq-mt: remove the /N for
thread case"), we do not need to decide the size by nr_task since this
is a thread mode test.

Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
diff --git a/case-anon-cow-rand-mt b/case-anon-cow-rand-mt
index caeb488..b47d810 100755
--- a/case-anon-cow-rand-mt
+++ b/case-anon-cow-rand-mt
@@ -2,4 +2,4 @@
 
 . ./hw_vars
 
-$USEMEM -t $nr_task --prealloc --prefault --random $((mem / nr_task / 2))
+$USEMEM -t $nr_task --prealloc --prefault --random $((mem / 2))