Fix a possible race between udp dispatch and socket code

There's a small possibility of race between udp dispatcher and
socket code - socket code can still hold internal reference to a
socket while dispatcher calls isc_socket_open, which can cause
an assertion failure. Fix it by relaxing the assertion test, and
instead simply locking the socket in isc_socket_open.

lib/isc/unix/socket.c

@@ -2596,15 +2596,16 @@ isc_socket_open(isc_socket_t *sock0) {
 
 	REQUIRE(VALID_SOCKET(sock));
 
-	REQUIRE(isc_refcount_current(&sock->references) == 1);
-	/*
-	 * We don't need to retain the lock hereafter, since no one else has
-	 * this socket.
-	 */
+	LOCK(&sock->lock);
+
+	REQUIRE(isc_refcount_current(&sock->references) >= 1);
 	REQUIRE(sock->fd == -1);
 	REQUIRE(sock->threadid == -1);
 
 	result = opensocket(sock->manager, sock, NULL);
+
+	UNLOCK(&sock->lock);
+
 	if (result != ISC_R_SUCCESS) {
 		sock->fd = -1;
 	} else {