mnt: Move ns_fd assignment down in prepare_mnt_ns()

No functional changes.
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/mount.c

@@ -3283,12 +3283,11 @@ int prepare_mnt_ns(void)
 				pr_perror("Can't restore mntns back");
 				goto err;
 			}
-			nsid->mnt.ns_fd = rst;
-			rst = fd;
-		} else {
+			SWAP(rst, fd);
+		}
+
 		/* Pin one with a file descriptor */
 		nsid->mnt.ns_fd = fd;
-		}
 
 		/* Set its root */
 		path[0] = '/';