docs: networking: convert l2tp.txt to ReST

- add SPDX header;
- add a document title;
- mark tables as such;
- adjust identation, whitespaces and blank lines;
- add to networking/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/index.rst

@@ -74,6 +74,7 @@ Contents:
    ipvlan
    ipvs-sysctl
    kcm
+   l2tp
 
 .. only::  subproject and html
 

Documentation/networking/l2tp.rst

@@ -1,3 +1,9 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+====
+L2TP
+====
+
 This document describes how to use the kernel's L2TP drivers to
 provide L2TP functionality. L2TP is a protocol that tunnels one or
 more sessions over an IP tunnel. It is commonly used for VPNs
@@ -121,14 +127,16 @@ Userspace may control behavior of the tunnel or session using
 setsockopt and ioctl on the PPPoX socket. The following socket
 options are supported:-
 
-DEBUG     - bitmask of debug message categories. See below.
+=========   ===========================================================
+DEBUG       bitmask of debug message categories. See below.
 SENDSEQ     - 0 => don't send packets with sequence numbers
-            1 => send packets with sequence numbers
+	    - 1 => send packets with sequence numbers
 RECVSEQ     - 0 => receive packet sequence numbers are optional
-            1 => drop receive packets without sequence numbers
+	    - 1 => drop receive packets without sequence numbers
 LNSMODE     - 0 => act as LAC.
-            1 => act as LNS.
-REORDERTO - reorder timeout (in millisecs). If 0, don't try to reorder.
+	    - 1 => act as LNS.
+REORDERTO   reorder timeout (in millisecs). If 0, don't try to reorder.
+=========   ===========================================================
 
 Only the DEBUG option is supported by the special tunnel management
 PPPoX socket.
@@ -177,18 +185,20 @@ setsockopt on the PPPoX socket to set a debug mask.
 
 The following debug mask bits are available:
 
+================  ==============================
 L2TP_MSG_DEBUG    verbose debug (if compiled in)
 L2TP_MSG_CONTROL  userspace - kernel interface
 L2TP_MSG_SEQ      sequence numbers handling
 L2TP_MSG_DATA     data packets
+================  ==============================
 
 If enabled, files under a l2tp debugfs directory can be used to dump
 kernel state about L2TP tunnels and sessions. To access it, the
-debugfs filesystem must first be mounted.
+debugfs filesystem must first be mounted::
 
 	# mount -t debugfs debugfs /debug
 
-Files under the l2tp directory can then be accessed.
+Files under the l2tp directory can then be accessed::
 
 	# cat /debug/l2tp/tunnels
 
@@ -211,7 +221,7 @@ iproute2's ip utility to support this.
 
 To create an L2TPv3 ethernet pseudowire between local host 192.168.1.1
 and peer 192.168.1.2, using IP addresses 10.5.1.1 and 10.5.1.2 for the
-tunnel endpoints:-
+tunnel endpoints::
 
 	# ip l2tp add tunnel tunnel_id 1 peer_tunnel_id 1 udp_sport 5000 \
 	  udp_dport 5000 encap udp local 192.168.1.1 remote 192.168.1.2
@@ -228,16 +238,19 @@ Repeat the above at the peer, with ports, tunnel/session ids and IP
 addresses reversed.  The tunnel and session IDs can be any non-zero
 32-bit number, but the values must be reversed at the peer.
 
+========================       ===================
 Host 1                         Host2
+========================       ===================
 udp_sport=5000                 udp_sport=5001
 udp_dport=5001                 udp_dport=5000
 tunnel_id=42                   tunnel_id=45
 peer_tunnel_id=45              peer_tunnel_id=42
 session_id=128                 session_id=5196755
 peer_session_id=5196755        peer_session_id=128
+========================       ===================
 
 When done at both ends of the tunnel, it should be possible to send
-data over the network. e.g.
+data over the network. e.g.::
 
 	# ping 10.5.1.1
 
@@ -245,7 +258,7 @@ data over the network. e.g.
 Sample Userspace Code
 =====================
 
-1. Create tunnel management PPPoX socket
+1. Create tunnel management PPPoX socket::
 
 	kernel_fd = socket(AF_PPPOX, SOCK_DGRAM, PX_PROTO_OL2TP);
 	if (kernel_fd >= 0) {
@@ -272,7 +285,7 @@ Sample Userspace Code
 		}
 	}
 
-2. Create session PPPoX data socket
+2. Create session PPPoX data socket::
 
 	struct sockaddr_pppol2tp sax;
 	int fd;