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: 26dd68d2 ("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>