treewide: Standardise variable names for various packet lengths

At various points we need to track the lengths of a packet including or
excluding various different sets of headers.  We don't always use the same
variable names for doing so.  Worse in some places we use the same name
for different things: e.g. tcp_fill_headers[46]() use ip_len for the
length including the IP headers, but then tcp_send_flag() which calls it
uses it to mean the IP payload length only.

To improve clarity, standardise on these names:
   dlen:		L4 protocol payload length ("data length")
   l4len:		plen + length of L4 protocol header
   l3len:		l4len + length of IPv4/IPv6 header
   l2len:		l3len + length of L2 (ethernet) header

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

pcap.c

@@ -79,30 +79,30 @@ struct pcap_pkthdr {
 static void pcap_frame(const struct iovec *iov, size_t iovcnt,
 		       size_t offset, const struct timespec *now)
 {
-	size_t len = iov_size(iov, iovcnt) - offset;
+	size_t l2len = iov_size(iov, iovcnt) - offset;
 	struct pcap_pkthdr h = {
 		.tv_sec = now->tv_sec,
 		.tv_usec = DIV_ROUND_CLOSEST(now->tv_nsec, 1000),
-		.caplen = len,
-		.len = len
+		.caplen = l2len,
+		.len = l2len
 	};
 	struct iovec hiov = { &h, sizeof(h) };
 
 	if (write_remainder(pcap_fd, &hiov, 1, 0) < 0 ||
 	    write_remainder(pcap_fd, iov, iovcnt, offset) < 0) {
 		debug("Cannot log packet, length %zu: %s",
-		      len, strerror(errno));
+		      l2len, strerror(errno));
 	}
 }
 
 /**
  * pcap() - Capture a single frame to pcap file
  * @pkt:	Pointer to data buffer, including L2 headers
- * @len:	L2 packet length
+ * @l2len:	L2 frame length
  */
-void pcap(const char *pkt, size_t len)
+void pcap(const char *pkt, size_t l2len)
 {
-	struct iovec iov = { (char *)pkt, len };
+	struct iovec iov = { (char *)pkt, l2len };
 	struct timespec now;
 
 	if (pcap_fd == -1)