netfilter: ebtables: replace zero-length array members

As suggested by Kees[1], replace the old-style 0-element array members
of multiple structs in ebtables.h with modern C99 flexible array.

[1]: https://lore.kernel.org/all/[e-mail]/

[ [e-mail]:
keep struct ebt_entry_target as-is, causes compiler warning:
"variable sized type 'struct ebt_entry_target' not at the end of a
struct or class is a GNU extension" ]

Link: https://github.com/KSPP/linux/issues/21
Signed-off-by: GONG, Ruiqi <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Signed-off-by: Florian Westphal <[e-mail]>

include/uapi/linux/netfilter_bridge/ebtables.h

···
 	/* nr. of entries */
 	unsigned int nentries;
 	/* entry list */
-	char data[0] __attribute__ ((aligned (__alignof__(struct ebt_replace))));
+	char data[] __attribute__ ((aligned (__alignof__(struct ebt_replace))));
 };
 
 /* used for the bitmask of struct ebt_entry */
···
 	} u;
 	/* size of data */
 	unsigned int match_size;
-	unsigned char data[0] __attribute__ ((aligned (__alignof__(struct ebt_replace))));
+	unsigned char data[] __attribute__ ((aligned (__alignof__(struct ebt_replace))));
 };
 
 struct ebt_entry_watcher {
···
 	} u;
 	/* size of data */
 	unsigned int watcher_size;
-	unsigned char data[0] __attribute__ ((aligned (__alignof__(struct ebt_replace))));
+	unsigned char data[] __attribute__ ((aligned (__alignof__(struct ebt_replace))));
 };
 
 struct ebt_entry_target {
···
 		/* sizeof ebt_entry + matches + watchers + target */
 		unsigned int next_offset;
 	);
-	unsigned char elems[0] __attribute__ ((aligned (__alignof__(struct ebt_replace))));
+	unsigned char elems[] __attribute__ ((aligned (__alignof__(struct ebt_replace))));
 };
 
 static __inline__ struct ebt_entry_target *