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-era-target.c

@@ -1635,7 +1635,8 @@ err:
 	DMEMIT("Error");
 }
 
-static int era_message(struct dm_target *ti, unsigned argc, char **argv)
+static int era_message(struct dm_target *ti, unsigned argc, char **argv,
+		       char *result, unsigned maxlen)
 {
 	struct era *era = ti->private;