bcachefs: Fix return code from bch2_fs_start()

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/super.c

@@ -897,7 +897,8 @@ int bch2_fs_start(struct bch_fs *c)
 		break;
 	}
 
-	BUG_ON(!ret);
+	if (ret >= 0)
+		ret = -EIO;
 	goto out;
 }