fuzz: unify logging setup

Make sure we don't log anything when running in "fuzzing" mode. Also,
when at it, unify the setup logic into a helper, pretty similar to
the test_setup_logging() one.

Addresses:
  - https://github.com/systemd/systemd/pull/29558#pullrequestreview-1676060607
  - https://github.com/systemd/systemd/pull/29558#discussion_r1358940663

src/network/fuzz-netdev-parser.c

@@ -14,8 +14,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         if (outside_size_range(size, 0, 65536))
                 return 0;
 
-        if (!getenv("SYSTEMD_LOG_LEVEL"))
-                log_set_max_level(LOG_CRIT);
+        fuzz_setup_logging();
 
         assert_se(fmkostemp_safe(netdev_config, "r+", &f) == 0);
         if (size != 0)