core: generate fixed fallback timestamp in _host_id_read_timestamp()

nm_hash_siphash42() uses a randomized seed like nm_hash*(). In this case,
we want to always generate the same fake timestamp, based on the host-id.

In practice, it doesn't really matter, because this is only the fallback
path for something gone horribly wrong already.

src/core/nm-core-utils.c

@@ -2711,7 +2711,7 @@ _host_id_read_timestamp(gboolean      use_secret_key_file,
 
 #define EPOCH_TWO_YEARS (G_GINT64_CONSTANT(2 * 365 * 24 * 3600) * NM_UTILS_NSEC_PER_SEC)
 
-    v = nm_hash_siphash42(1156657133u, host_id, host_id_len);
+    v = c_siphash_hash(NM_HASH_SEED_16_U64(1156657133u), host_id, host_id_len);
 
     now = time(NULL);
     *out_timestamp_ns =

src/libnm-glib-aux/nm-hash-utils.h

@@ -56,6 +56,9 @@ void nm_hash_siphash42_init(CSipHash *h, guint static_seed);
  * Then, why not use c_siphash_hash() directly? Because this also uses the randomized,
  * per-run hash-seed like nm_hash_init(). So, you get siphash24 with a random
  * seed (which is cached for the current run of the program).
+ *
+ * WARNING: the static_seed gets randomized like with nm_hash*(). If you want a reproducible
+ *   siphash42, use instead `c_siphash_hash(NM_HASH_SEED_16_U64(number), ptr, len)`.
  */
 static inline guint64
 nm_hash_siphash42(guint static_seed, const void *ptr, gsize n)