protect: move us_heap to userspace_data_s

Refactor heap pointer storage by moving us_heap field from userspace_s
to the nested userspace_data_s structure, enabling future extensibility
of user-space data without modifying the core userspace_s interface across
all board-specific implementations.

Signed-off-by: hujun5 <hujun5@xiaomi.com>

include/nuttx/mm/mm.h

@@ -124,7 +124,7 @@
  * structure from the userspace interface.
  */
 
-#  define USR_HEAP (*USERSPACE->us_heap)
+#  define USR_HEAP (*USERSPACE->us_data->us_heap)
 
 #else
 /* Otherwise, the user heap data structures are in common .bss */

include/nuttx/userspace.h

@@ -77,6 +77,11 @@
 
 struct mm_heap_s; /* Forward reference */
 
+struct userspace_data_s
+{
+  FAR struct mm_heap_s **us_heap;
+};
+
 /* Every user-space blob starts with a header that provides information about
  * the blob.  The form of that header is provided by struct userspace_s. An
  * instance of this is expected to reside at CONFIG_NUTTX_USERSPACE.
@@ -96,9 +101,9 @@ struct userspace_s
   uintptr_t us_bssend;
   uintptr_t us_heapend;
 
-  /* Memory manager heap structure */
+  /* User data memory structure */
 
-  FAR struct mm_heap_s **us_heap;
+  FAR struct userspace_data_s *us_data;
 
   /* Task startup routine */