Add variant of close_all_fds() that does not allocate and use it in freeze()

Even though it's just a fallback path, let's not be sloppy and allocate in
the crash handler.

> The deadlock happens because systemd crash in malloc() then in signal
> handler, it calls malloc() (close_all_fds()-> opendir()-> __alloc_dir())
> again. malloc() is not a signal-safe function, maybe we should re-think
> the logic here.

Fixes #20266.

src/basic/fd-util.h

@@ -56,7 +56,10 @@ DEFINE_TRIVIAL_CLEANUP_FUNC_FULL(DIR*, closedir, NULL);
 int fd_nonblock(int fd, bool nonblock);
 int fd_cloexec(int fd, bool cloexec);
 
-int close_all_fds(int except[], size_t n_except);
+int close_all_fds_full(int except[], size_t n_except, bool allow_alloc);
+static inline int close_all_fds(int except[], size_t n_except) {
+        return close_all_fds_full(except, n_except, true);
+}
 
 int same_fd(int a, int b);