Move I_P_List_adapter to sql_plist.h as it is currently being used

for code other then in mdl.h. Since the adapter is generic, it is
better located in sql_plist.h.

sql/mdl.h

@@ -286,21 +286,6 @@ class MDL_key
 };
 
 
-
-/**
-  Hook class which via its methods specifies which members
-  of T should be used for participating in MDL lists.
-*/
-
-template <typename T, T* T::*next, T** T::*prev>
-struct I_P_List_adapter
-{
-  static inline T **next_ptr(T *el) { return &(el->*next); }
-
-  static inline T ***prev_ptr(T *el) { return &(el->*prev); }
-};
-
-
 /**
   A pending metadata lock request.
 

sql/sql_plist.h

@@ -187,6 +187,20 @@ class I_P_List_iterator
 };
 
 
+/**
+  Hook class which via its methods specifies which members
+  of T should be used for participating in a intrusive list.
+*/
+
+template <typename T, T* T::*next, T** T::*prev>
+struct I_P_List_adapter
+{
+  static inline T **next_ptr(T *el) { return &(el->*next); }
+
+  static inline T ***prev_ptr(T *el) { return &(el->*prev); }
+};
+
+
 /**
   Element counting policy class for I_P_List to be used in
   cases when no element counting should be done.