keys: Annotate struct asymmetric_key_id with __counted_by

Add the __counted_by() compiler attribute to the flexible array member
'data' to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
CONFIG_FORTIFY_SOURCE.

Signed-off-by: Thorsten Blum <[e-mail]>
Reviewed-by: Lukas Wunner <[e-mail]>
Reviewed-by: Jarkko Sakkinen <[e-mail]>
Signed-off-by: Herbert Xu <[e-mail]>

include/keys/asymmetric-type.h

···
  */
 struct asymmetric_key_id {
 	unsigned short	len;
-	unsigned char	data[];
+	unsigned char	data[] __counted_by(len);
 };
 
 struct asymmetric_key_ids {