dm writecache: report invalid return from writecache_map helpers

If some "writecache_map_*" function returns invalid state, it is a bug.
So, we should report it and not fail silently.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-writecache.c

@@ -1545,10 +1545,13 @@ done:
 		return DM_MAPIO_SUBMITTED;
 
 	case WC_MAP_ERROR:
-	default:
 		wc_unlock(wc);
 		bio_io_error(bio);
 		return DM_MAPIO_SUBMITTED;
+
+	default:
+		BUG();
+		return -1;
 	}
 }