dm writecache: fix flexible_array.cocci warnings

Zero-length and one-element arrays are deprecated, see
Documentation/process/deprecated.rst
Flexible-array members should be used instead.

Generated by: scripts/coccinelle/misc/flexible_array.cocci

CC: Denis Efremov <[e-mail]>
Reported-by: kernel test robot <[e-mail]>
Signed-off-by: kernel test robot <[e-mail]>
Signed-off-by: Julia Lawall <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/dm-writecache.c

···
 		};
 		__le64 padding[8];
 	};
-	struct wc_memory_entry entries[0];
+	struct wc_memory_entry entries[];
 };
 
 struct wc_entry {