w1: Avoid -Wflex-array-member-not-at-end warnings

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Use the `DEFINE_RAW_FLEX()` helper for on-stack definitions of
a flexible structure where the size of the flexible-array member
is known at compile-time, and refactor the rest of the code,
accordingly.

So, with these changes, fix the following warnings:

drivers/w1/w1_netlink.c:198:31: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
drivers/w1/w1_netlink.c:219:31: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/Z_RflBe5iDGTMFjV@kspp


Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250513105326.27385-2-krzysztof.kozlowski@linaro.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>