MINOR: dev/udp: Apply the corruption to both directions

Harden the UDP datagram corruption applying it on both sides. This approaches
the conditions of some tests run by the QUIC interop runner developed by
Marten Seeman.

dev/udp/udp-perturb.c

@@ -275,6 +275,18 @@ int add_connection(struct sockaddr_storage *ss)
 	return -1;
 }
 
+/* Corrupt <buf> buffer with <buflen> as length if required */
+static void pktbuf_apply_corruption(char *buf, size_t buflen)
+{
+	if (corr_rate > 0 && prng(100) < corr_rate) {
+		unsigned int rnd = prng(corr_span * 256); // pos and value
+		unsigned int pos = corr_base + (rnd >> 8);
+
+		if (pos < buflen)
+			buf[pos] ^= rnd;
+	}
+}
+
 /* Handle a read operation on an front FD. Will either reuse the existing
  * connection if the source is found, or will allocate a new one, possibly
  * replacing the oldest one. Returns <0 on error or the number of bytes
@@ -323,13 +335,7 @@ int handle_frt(int fd, struct pollfd *pfd, struct conn *conns, int nbconn)
 	if (ret < 0)
 		return errno == EAGAIN ? 0 : -1;
 
-	if (corr_rate > 0 && prng(100) < corr_rate) {
-		unsigned int rnd = prng(corr_span * 256); // pos and value
-		unsigned int pos = corr_base + (rnd >> 8);
-
-		if (pos < ret)
-			pktbuf[pos] ^= rnd;
-	}
+	pktbuf_apply_corruption(pktbuf, ret);
 
 	conn = NULL;
 	for (i = 0; i < nbconn; i++) {
@@ -411,6 +417,8 @@ int handle_bck(int fd, struct pollfd *pfd, struct conn *conns, int nbconn)
 	if (ret < 0)
 		return errno == EAGAIN ? 0 : -1;
 
+	pktbuf_apply_corruption(pktbuf, ret);
+
 	conn = conn_bck_lookup(conns, nbconn, fd);
 	if (!conn)
 		return 0;