mm_size2ndx: using flsl to calculate the ndx

Signed-off-by: wangbowen6 <wangbowen6@xiaomi.com>

mm/mm_heap/mm_size2ndx.c

@@ -24,6 +24,8 @@
 
 #include <nuttx/config.h>
 
+#include <string.h>
+
 #include <nuttx/mm/mm.h>
 
 #include "mm_heap/mm.h"
@@ -42,19 +44,11 @@
 
 int mm_size2ndx(size_t size)
 {
-  int ndx = 0;
-
   if (size >= MM_MAX_CHUNK)
     {
       return MM_NNODES - 1;
     }
 
   size >>= MM_MIN_SHIFT;
-  while (size > 1)
-    {
-      ndx++;
-      size >>= 1;
-    }
-
-  return ndx;
+  return flsl(size) - 1;
 }