stddef: Introduce __TRAILING_OVERLAP()

Introduce underlying __TRAILING_OVERLAP() macro to let callers apply
atributes to trailing overlapping members.

For instance, the code below:

| struct flex {
| size_t count;
| int data[];
| };

| struct {
| struct flex f;
| struct foo a;
| struct boo b;
| } __packed instance;

can now be changed to the following, and preserve the __packed
attribute:

| __TRAILING_OVERLAP(struct flex, f, data, __packed,
| struct foo a;
| struct boo b;
| ) instance;

Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Link: https://lore.kernel.org/r/f8[e-mail]
Signed-off-by: Kees Cook <[e-mail]>

include/linux/stddef.h

···
 	__DECLARE_FLEX_ARRAY(TYPE, NAME)
 
 /**
+ * __TRAILING_OVERLAP() - Overlap a flexible-array member with trailing
+ *			  members.
+ *
+ * Creates a union between a flexible-array member (FAM) in a struct and a set
+ * of additional members that would otherwise follow it.
+ *
+ * @TYPE: Flexible structure type name, including "struct" keyword.
+ * @NAME: Name for a variable to define.
+ * @FAM: The flexible-array member within @TYPE
+ * @ATTRS: Any struct attributes (usually empty)
+ * @MEMBERS: Trailing overlapping members.
+ */
+#define __TRAILING_OVERLAP(TYPE, NAME, FAM, ATTRS, MEMBERS)			\
+	union {									\
+		TYPE NAME;							\
+		struct {							\
+			unsigned char __offset_to_FAM[offsetof(TYPE, FAM)];	\
+			MEMBERS							\
+		} ATTRS;							\
+	}
+
+/**
  * TRAILING_OVERLAP() - Overlap a flexible-array member with trailing members.
  *
  * Creates a union between a flexible-array member (FAM) in a struct and a set
···
  * @MEMBERS: Trailing overlapping members.
  */
 #define TRAILING_OVERLAP(TYPE, NAME, FAM, MEMBERS)				\
-	union {									\
-		TYPE NAME;							\
-		struct {							\
-			unsigned char __offset_to_FAM[offsetof(TYPE, FAM)];	\
-			MEMBERS							\
-		};								\
-	}
+	__TRAILING_OVERLAP(TYPE, NAME, FAM, /* no attrs */, MEMBERS)
 
 #endif