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: 26dd68d293fd ("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>