chage memlaout USTACK_LIMIT

kernel/memlayout.h

@@ -73,7 +73,7 @@
 #define HEAP_THRESHOLD (8*1024*1024)
 
 #define USTACK_BOTTOM (256*1024*1024)
-#define USTACK_LIMIT (4 * 1024)
+#define USTACK_LIMIT (256 * 1024)
 #define USTACK_TOP (USTACK_BOTTOM + USTACK_LIMIT)
 
 #endif