docs: networking: convert nf_flowtable.txt to ReST

- add SPDX header;
- adjust title markup;
- mark code blocks and literals as such;
- add notes markups;
- 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

@@ -86,6 +86,7 @@ Contents:
    netfilter-sysctl
    netif-msg
    nf_conntrack-sysctl
+   nf_flowtable
 
 .. only::  subproject and html
 

Documentation/networking/nf_flowtable.rst

@@ -1,3 +1,6 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+====================================
 Netfilter's flowtable infrastructure
 ====================================
 
@@ -31,6 +34,8 @@ to use this new alternative forwarding path via nftables policy.
 This is represented in Fig.1, which describes the classic forwarding path
 including the Netfilter hooks and the flowtable fastpath bypass.
 
+::
+
 					 userspace process
 					  ^              |
 					  |              |
@@ -72,7 +77,7 @@ Example configuration
 ---------------------
 
 Enabling the flowtable bypass is relatively easy, you only need to create a
-flowtable and add one rule to your forward chain.
+flowtable and add one rule to your forward chain::
 
 	table inet x {
 		flowtable f {
@@ -101,12 +106,12 @@ forwarding bypass.
 More reading
 ------------
 
-This documentation is based on the LWN.net articles [1][2]. Rafal Milecki also
-made a very complete and comprehensive summary called "A state of network
+This documentation is based on the LWN.net articles [1]_\ [2]_. Rafal Milecki
+also made a very complete and comprehensive summary called "A state of network
 acceleration" that describes how things were before this infrastructure was
-mailined [3] and it also makes a rough summary of this work [4].
+mailined [3]_ and it also makes a rough summary of this work [4]_.
 
-[1] https://lwn.net/Articles/738214/
-[2] https://lwn.net/Articles/742164/
-[3] http://lists.infradead.org/pipermail/lede-dev/2018-January/010830.html
-[4] http://lists.infradead.org/pipermail/lede-dev/2018-January/010829.html
+.. [1] https://lwn.net/Articles/738214/
+.. [2] https://lwn.net/Articles/742164/
+.. [3] http://lists.infradead.org/pipermail/lede-dev/2018-January/010830.html
+.. [4] http://lists.infradead.org/pipermail/lede-dev/2018-January/010829.html