md/bcache: Mark __nonstring look-up table

GCC 15's new -Wunterminated-string-initialization notices that the 16
character lookup table "zero_uuid" (which is not used as a C-String)
needs to be marked as "nonstring":

drivers/md/bcache/super.c: In function 'uuid_find_empty':
drivers/md/bcache/super.c:549:43: warning: initializer-string for array of 'char' truncates NUL terminator but destination lacks 'nonstring' attribute (17 chars into 16 available) [-Wunterminated-string-initialization]
549 | static const char zero_uuid[16] = "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0";
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Add the annotation (since it is not used as a C-String), and switch the
initializer to an array of bytes rather than an empty initializer,
as preferred by Coly Li.

Suggested-by: Coly Li <colyli@kernel.org>
Link: https://lore.kernel.org/lkml/389A9925-0990-422C-A1B3-0195FAA73288@coly.li/
Signed-off-by: Kees Cook <kees@kernel.org>