ThreadSanitizer: data race in zonemgr_keymgmt_delete()

See this failed job: https://gitlab.isc.org/isc-projects/bind9/-/jobs/2982394

The TSAN report is here: tsan.named.32229.

This was most likely uncovered after #3727 (closed) was fixed, as before that the code under the "Delete the entry" comment in zone.c:zonemgr_keymgmt_delete() (see below) wasn't being executed:

		if (dns_name_equal(kfio->name, &zone->origin)) {
			unsigned int count;

			count = atomic_fetch_sub_relaxed(&kfio->count, 1) - 1;
			if (count > 0) {
				/* Keep the entry. */
				break;
			}

			/* Delete the entry. */
			if (prev == NULL) {
				mgmt->table[hash] = kfio->next;
			} else {
				prev->next = kfio->next;
			}

			isc_mutex_destroy(&kfio->lock);
			isc_mem_put(mgmt->mctx, kfio, sizeof(*kfio));

			atomic_fetch_sub_relaxed(&mgmt->count, 1); // <---- the report is here

			break;
		}