move MEM_ROOT::read_only into flags

include/my_alloc.h

@@ -23,6 +23,8 @@
 #define ALLOC_MAX_BLOCK_TO_DROP			4096
 #define ALLOC_MAX_BLOCK_USAGE_BEFORE_DROP	10
 
+#define ROOT_FLAG_READ_ONLY       4
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -52,10 +54,6 @@ typedef struct st_mem_root
   unsigned short first_block_usage;
   unsigned short flags;
 
-#ifdef PROTECT_STATEMENT_MEMROOT
-  int read_only;
-#endif
-
   void (*error_handler)(void);
   const char *name;
 } MEM_ROOT;