vboxsf: 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 arrays
with __nonstring to and correctly identify the char array as "not a C
string" and thereby eliminate the warning.

This effectively reverts the change in 4e74872 ("vboxsf: fix building
with GCC 15"), to add the annotation that has other uses (i.e. warning
if the string is ever used with C string APIs).

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117178 [1]
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Brahmajit Das <brahmajit.xyz@gmail.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>
Link: https://lore.kernel.org/r/20250310222530.work.374-kees@kernel.org
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: kees

fs/vboxsf/super.c

@@ -21,8 +21,7 @@
 
 #define VBOXSF_SUPER_MAGIC 0x786f4256 /* 'VBox' little endian */
 
-static const unsigned char VBSF_MOUNT_SIGNATURE[4] = { '\000', '\377', '\376',
-						       '\375' };
+static const unsigned char VBSF_MOUNT_SIGNATURE[4] __nonstring = "\000\377\376\375";
 
 static int follow_symlinks;
 module_param(follow_symlinks, int, 0444);