bcache: Replace zero-length arrays with DECLARE_FLEX_ARRAY() helper

Zero-length arrays are deprecated and we are moving towards adopting
C99 flexible-array members, instead. So, replace zero-length arrays
declarations in anonymous union with the new DECLARE_FLEX_ARRAY()
helper macro.

This helper allows for flexible-array members in unions.

Link: KSPP#193
Link: KSPP#213
Link: https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

Author: GustavoARSilva

drivers/md/bcache/bcache_ondisk.h

@@ -359,8 +359,8 @@ struct jset {
 	__u64			prio_bucket[MAX_CACHES_PER_SET];
 
 	union {
-		struct bkey	start[0];
-		__u64		d[0];
+		DECLARE_FLEX_ARRAY(struct bkey, start);
+		DECLARE_FLEX_ARRAY(__u64, d);
 	};
 };
 
@@ -424,8 +424,8 @@ struct bset {
 	__u32			keys;
 
 	union {
-		struct bkey	start[0];
-		__u64		d[0];
+		DECLARE_FLEX_ARRAY(struct bkey, start);
+		DECLARE_FLEX_ARRAY(__u64, d);
 	};
 };