overflow: Fix direct struct member initialization in _DEFINE_FLEX()

Currently, to statically initialize the struct members of the `type`
object created by _DEFINE_FLEX(), the internal `obj` member must be
explicitly referenced at the call site. See:

struct flex {
        int a;
        int b;
        struct foo flex_array[];
};

_DEFINE_FLEX(struct flex, instance, flex_array,
                 FIXED_SIZE, = {
                        .obj = {
                                .a = 0,
                                .b = 1,
                        },
                });

This leaks _DEFINE_FLEX() internal implementation details and make
the helper harder to use and read.

Fix this and allow for a more natural and intuitive C99 init-style:

_DEFINE_FLEX(struct flex, instance, flex_array,
                 FIXED_SIZE, = {
                        .a = 0,
                        .b = 1,
                });

Note that before these changes, the `initializer` argument was optional,
but now it's required.

Also, update "counter" member initialization in DEFINE_FLEX().

Fixes: 26dd68d ("overflow: add DEFINE_FLEX() for on-stack allocs")
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/aBQVeyKfLOkO9Yss@kspp
Signed-off-by: Kees Cook <kees@kernel.org>

Author: GustavoARSilva

include/linux/overflow.h

@@ -396,15 +396,15 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
  * @name: Name for a variable to define.
  * @member: Name of the array member.
  * @count: Number of elements in the array; must be compile-time const.
- * @initializer: initializer expression (could be empty for no init).
+ * @initializer: Initializer expression (e.g., pass `= { }` at minimum).
  */
 #define _DEFINE_FLEX(type, name, member, count, initializer...)			\
 	_Static_assert(__builtin_constant_p(count),				\
 		       "onstack flex array members require compile-time const count"); \
 	union {									\
 		u8 bytes[struct_size_t(type, member, count)];			\
 		type obj;							\
-	} name##_u initializer;							\
+	} name##_u = { .obj initializer };					\
 	type *name = (type *)&name##_u
 
 /**
@@ -444,7 +444,7 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
  * elements in array @member.
  */
 #define DEFINE_FLEX(TYPE, NAME, MEMBER, COUNTER, COUNT)	\
-	_DEFINE_FLEX(TYPE, NAME, MEMBER, COUNT, = { .obj.COUNTER = COUNT, })
+	_DEFINE_FLEX(TYPE, NAME, MEMBER, COUNT, = { .COUNTER = COUNT, })
 
 /**
  * STACK_FLEX_ARRAY_SIZE() - helper macro for DEFINE_FLEX() family.