docs: networking: convert seg6-sysctl.txt to ReST

- add SPDX header;
- mark code blocks and literals as such;
- add a document title;
- adjust chapters, adding proper markups;
- mark lists as such;
- add to networking/index.rst.

Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/networking/index.rst

···
    rxrpc
    sctp
    secid
+   seg6-sysctl
 
 .. only::  subproject and html
 

Documentation/networking/seg6-sysctl.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
+====================
+Seg6 Sysfs variables
+====================
+
+
+/proc/sys/net/conf/<iface>/seg6_* variables:
+============================================
+
+seg6_enabled - BOOL
+	Accept or drop SR-enabled IPv6 packets on this interface.
+
+	Relevant packets are those with SRH present and DA = local.
+
+	* 0 - disabled (default)
+	* not 0 - enabled
+
+seg6_require_hmac - INTEGER
+	Define HMAC policy for ingress SR-enabled packets on this interface.
+
+	* -1 - Ignore HMAC field
+	* 0 - Accept SR packets without HMAC, validate SR packets with HMAC
+	* 1 - Drop SR packets without HMAC, validate SR packets with HMAC
+
+	Default is 0.

Documentation/networking/seg6-sysctl.txt

···
-/proc/sys/net/conf/<iface>/seg6_* variables:
-
-seg6_enabled - BOOL
-	Accept or drop SR-enabled IPv6 packets on this interface.
-
-	Relevant packets are those with SRH present and DA = local.
-
-	0 - disabled (default)
-	not 0 - enabled
-
-seg6_require_hmac - INTEGER
-	Define HMAC policy for ingress SR-enabled packets on this interface.
-
-	-1 - Ignore HMAC field
-	0 - Accept SR packets without HMAC, validate SR packets with HMAC
-	1 - Drop SR packets without HMAC, validate SR packets with HMAC
-
-	Default is 0.