Add lower threshold for multithreading in ?potrf and ?potri

interface/lapack/zpotrf.c

@@ -112,6 +112,13 @@ int NAME(char *UPLO, blasint *N, FLOAT *a, blasint *ldA, blasint *Info){
 
 #ifdef SMP
   args.common = NULL;
+#ifndef DOUBLE
+  if (args.n < 64)
+#else
+  if (args.n < 64)
+#endif
+    args.nthreads = 1;
+  else
   args.nthreads = num_cpu_avail(4);
 
   if (args.nthreads == 1) {