dm mpath: complain about unsupported __multipath_map_bio() return values

WARN_ONCE() if __multipath_map_bio() returns an unsupported return value.

Signed-off-by: Bart Van Assche <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/dm-mpath.c

···
 		case DM_MAPIO_REMAPPED:
 			generic_make_request(bio);
 			break;
+		case 0:
+			break;
+		default:
+			WARN_ONCE(true, "__multipath_map_bio() returned %d\n", r);
 		}
 	}
 	blk_finish_plug(&plug);