file_utils: harden lxc_writeat()

src/lxc/file_utils.c

@@ -47,8 +47,7 @@ int lxc_writeat(int dirfd, const char *filename, const void *buf, size_t count)
 	__do_close int fd = -EBADF;
 	ssize_t ret;
 
-	fd = openat(dirfd, filename,
-		    O_WRONLY | O_CLOEXEC | O_NOCTTY | O_NOFOLLOW);
+	fd = open_at(dirfd, filename, PROTECT_OPEN_W_WITH_TRAILING_SYMLINKS, PROTECT_LOOKUP_BENEATH, 0);
 	if (fd < 0)
 		return -1;