docs: networking: convert xfrm_sysctl.txt to ReST

Not much to be done here:

- add SPDX header;
- add a document title;
- add a chapter's markup;
- 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

@@ -120,6 +120,7 @@ Contents:
    xfrm_device
    xfrm_proc
    xfrm_sync
+   xfrm_sysctl
 
 .. only::  subproject and html
 

Documentation/networking/xfrm_sysctl.rst

@@ -1,4 +1,11 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+============
+XFRM Syscall
+============
+
 /proc/sys/net/core/xfrm_* Variables:
+====================================
 
 xfrm_acq_expires - INTEGER
 	default 30 - hard timeout in seconds for acquire requests