alloc_tag: avoid implicit padding in uapi

The implied padding causes a harmless warning when testing the uapi
headers with -Wpadded that could in theory indicate incompatibilities
or data leaks:

./usr/include/linux/alloc_tag.h:41:1: error: padding struct size to alignment boundary with 7 bytes [-Werror=padded]

The code here is fine, but it's better to make the padding explicit and
avoid the warning here.

Link: https://lore.kernel.org/20260916065830.1619425-1-arnd@kernel.org
Link: https://lore.kernel.org/20260915202404.3568029-1-arnd@kernel.org
Fixes: 1d581ab2348c ("alloc_tag: add ioctl to /proc/allocinfo")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Suren Baghdasaryan <surenb@google.com>
Acked-by: SJ Park <sj@kernel.org>
Acked-by: Hao Ge <hao.ge@linux.dev>