namespace: Clone root dir descriptor before use

Before doing anything with the root directory file descriptor, let's
make sure we clone it first so that the caller can't mess with mount fd
attributes via mount_setattr() anymore.

We clone during parsing instead of in executor so that the caller can't
mess with the mount fd between invocations.

src/core/execute.c

@@ -2877,6 +2877,7 @@ void exec_params_shallow_clear(ExecParameters *p) {
         p->fd_names = strv_free(p->fd_names);
         p->files_env = strv_free(p->files_env);
         p->fds = mfree(p->fds);
+        p->root_directory_fd = safe_close(p->root_directory_fd);
         p->exec_fd = safe_close(p->exec_fd);
         p->user_lookup_fd = -EBADF;
         p->bpf_restrict_fs_map_fd = -EBADF;