docs: filesystems: convert mandatory-locking.txt to ReST

- Add a SPDX header;
- Adjust document title;
- Some whitespace fixes and new line breaks;
- Use notes markups;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/aecd6259fe9f99b2c2b3440eab6a2b989125e00d.1588021877.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/index.rst

@@ -29,6 +29,7 @@ algorithms work.
    fiemap
    files
    locks
+   mandatory-locking
 
    automount-support
 

Documentation/filesystems/locks.rst

@@ -58,7 +58,7 @@ fcntl(), with all the problems that implies.
 ---------------------------------------
 
 Mandatory locking, as described in
-'Documentation/filesystems/mandatory-locking.txt' was prior to this release a
+'Documentation/filesystems/mandatory-locking.rst' was prior to this release a
 general configuration option that was valid for all mounted filesystems.  This
 had a number of inherent dangers, not the least of which was the ability to
 freeze an NFS server by asking it to read a file for which a mandatory lock

Documentation/filesystems/mandatory-locking.rst

@@ -1,8 +1,13 @@
-	Mandatory File Locking For The Linux Operating System
+.. SPDX-License-Identifier: GPL-2.0
+
+=====================================================
+Mandatory File Locking For The Linux Operating System
+=====================================================
 
 		Andy Walker <andy@lysaker.kvaerner.no>
 
 			   15 April 1996
+
 		     (Updated September 2007)
 
 0. Why you should avoid mandatory locking
@@ -53,15 +58,17 @@ possible on existing user code. The scheme is based on marking individual files
 as candidates for mandatory locking, and using the existing fcntl()/lockf()
 interface for applying locks just as if they were normal, advisory locks.
 
-Note 1: In saying "file" in the paragraphs above I am actually not telling
-the whole truth. System V locking is based on fcntl(). The granularity of
-fcntl() is such that it allows the locking of byte ranges in files, in addition
-to entire files, so the mandatory locking rules also have byte level
-granularity.
+.. Note::
 
-Note 2: POSIX.1 does not specify any scheme for mandatory locking, despite
-borrowing the fcntl() locking scheme from System V. The mandatory locking
-scheme is defined by the System V Interface Definition (SVID) Version 3.
+   1. In saying "file" in the paragraphs above I am actually not telling
+      the whole truth. System V locking is based on fcntl(). The granularity of
+      fcntl() is such that it allows the locking of byte ranges in files, in
+      addition to entire files, so the mandatory locking rules also have byte
+      level granularity.
+
+   2. POSIX.1 does not specify any scheme for mandatory locking, despite
+      borrowing the fcntl() locking scheme from System V. The mandatory locking
+      scheme is defined by the System V Interface Definition (SVID) Version 3.
 
 2. Marking a file for mandatory locking
 ---------------------------------------

fs/locks.c

@@ -61,7 +61,7 @@
  *
  *  Initial implementation of mandatory locks. SunOS turned out to be
  *  a rotten model, so I implemented the "obvious" semantics.
- *  See 'Documentation/filesystems/mandatory-locking.txt' for details.
+ *  See 'Documentation/filesystems/mandatory-locking.rst' for details.
  *  Andy Walker (andy@lysaker.kvaerner.no), April 06, 1996.
  *
  *  Don't allow mandatory locks on mmap()'ed files. Added simple functions to