uapi: stddef.h: Introduce __kernel_nonstring

In order to annotate byte arrays in UAPI that are not C strings (i.e.
they may not be NUL terminated), the "nonstring" attribute is needed.
However, we can't expose this to userspace as it is compiler version
specific.

Signed-off-by: Kees Cook <[e-mail]>

include/uapi/linux/stddef.h

···
 #define __counted_by_be(m)
 #endif
 
+#ifdef __KERNEL__
+#define __kernel_nonstring	__nonstring
+#else
+#define __kernel_nonstring
+#endif
+
 #endif /* _UAPI_LINUX_STDDEF_H */