Fix data race in offloaded dns_message_checksig()

When verifying a message in an offloaded thread there is a race with
the worker thread which writes to the same buffer. Clone the message
buffer before offloading.

(cherry picked from commit 35ef25e5ea)

lib/dns/message.c

@@ -3254,6 +3254,7 @@ dns_message_checksig_async(dns_message_t *msg, dns_view_t *view,
 	dns_message_attach(msg, &chsigctx->msg);
 	dns_view_attach(view, &chsigctx->view);
 
+	dns_message_clonebuffer(msg);
 	isc_work_enqueue(loop, checksig_run, checksig_cb, chsigctx);
 
 	return (DNS_R_WAIT);