cachefiles: Add __nonstring annotations for unterminated strings

When a character array without a terminating NUL character has a static initializer, GCC 15's -Wunterminated-string-initialization will only warn if the array lacks the "nonstring" attribute[1]. Mark the array with __nonstring to correctly identify the char array as "not a C string" and thereby eliminate the warning:

../fs/cachefiles/key.c:12:9: warning: initializer-string for array of 'char' truncates NUL terminator but destination lacks 'nonstring' attribute (65 chars into 64 available) [-Wunterminated-string-initialization]

   12 |         "0123456789"                    /* 0 - 9 */
      |         ^~~~~~~~~~~~

Since the array is used only for single character look-ups, use __nonstring:

fs/cachefiles/key.c:123:                name[len++] = cachefiles_charmap[acc & 63];
fs/cachefiles/key.c:125:                name[len++] = cachefiles_charmap[acc & 63];
fs/cachefiles/key.c:127:                name[len++] = cachefiles_charmap[acc & 63];
fs/cachefiles/key.c:129:                name[len++] = cachefiles_charmap[acc & 63];

---
v2: improve commit log
v1: https://lore.kernel.org/lkml/[email protected]/
Cc: David Howells <[email protected]>
Cc: [email protected]

---
 fs/cachefiles/key.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index bf935e25bdbe..b48525680e73 100644
@@ -8,7 +8,7 @@
 #include <linux/slab.h>
 #include "internal.h"
 
-static const char cachefiles_charmap[64] =
+static const char cachefiles_charmap[64] __nonstring =
 	"0123456789"			/* 0 - 9 */
 	"abcdefghijklmnopqrstuvwxyz"	/* 10 - 35 */
 	"ABCDEFGHIJKLMNOPQRSTUVWXYZ"	/* 36 - 61 */

2.34.1