dm: allow targets to return output from messages they are sent

Could be useful for a target to return stats or other information.
If a target does DMEMIT() anything to @result from its .message method
then it must return 1 to the caller.

Signed-off-By: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-cache-target.c

@@ -3387,7 +3387,8 @@ static int process_invalidate_cblocks_message(struct cache *cache, unsigned coun
  *
  * The key migration_threshold is supported by the cache target core.
  */
-static int cache_message(struct dm_target *ti, unsigned argc, char **argv)
+static int cache_message(struct dm_target *ti, unsigned argc, char **argv,
+			 char *result, unsigned maxlen)
 {
 	struct cache *cache = ti->private;