docs: fs: convert docs without extension to ReST

There are 3 remaining files without an extension inside the fs docs
dir.

Manually convert them to ReST.

In the case of the nfs/exporting.rst file, as the nfs docs
aren't ported yet, I opted to convert and add a :orphan: there,
with should be removed when it gets added into a nfs-specific
part of the fs documentation.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

fs/orangefs/file.c

@@ -555,7 +555,7 @@ static int orangefs_fsync(struct file *file,
  * Change the file pointer position for an instance of an open file.
  *
  * \note If .llseek is overriden, we must acquire lock as described in
- *       Documentation/filesystems/Locking.
+ *       Documentation/filesystems/locking.rst.
  *
  * Future upgrade could support SEEK_DATA and SEEK_HOLE but would
  * require much changes to the FS