docs: filesystems: convert locks.txt to ReST

- Add a SPDX header;
- Adjust document and section titles;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/467b3f20e63d2640d22599b99229699b5fb79251.1588021877.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/index.rst

@@ -28,6 +28,7 @@ algorithms work.
    dnotify
    fiemap
    files
+   locks
 
    automount-support
 

Documentation/filesystems/locks.rst

@@ -1,4 +1,8 @@
-		      File Locking Release Notes
+.. SPDX-License-Identifier: GPL-2.0
+
+==========================
+File Locking Release Notes
+==========================
 
 		Andy Walker <andy@lysaker.kvaerner.no>
 
@@ -6,7 +10,7 @@
 
 
 1. What's New?
---------------
+==============
 
 1.1 Broken Flock Emulation
 --------------------------
@@ -25,7 +29,7 @@ anyway (see the file "Documentation/process/changes.rst".)
 ---------------------------
 
 1.2.1 Typical Problems - Sendmail
----------------------------------
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 Because sendmail was unable to use the old flock() emulation, many sendmail
 installations use fcntl() instead of flock(). This is true of Slackware 3.0
 for example. This gave rise to some other subtle problems if sendmail was
@@ -37,7 +41,7 @@ to lock solid with deadlocked processes.
 
 
 1.2.2 The Solution
-------------------
+^^^^^^^^^^^^^^^^^^
 The solution I have chosen, after much experimentation and discussion,
 is to make flock() and fcntl() locks oblivious to each other. Both can
 exists, and neither will have any effect on the other.