docs: filesystems: vfs: Convert vfs.txt to RST

vfs.txt is currently stale.  If we convert it to RST this is a good
first step in the process of getting the VFS documentation up to date.

This patch does the following (all as a single patch so as not to
introduce any new SPHINX build warnings)

 - Use '.. code-block:: c' for C code blocks and indent the code blocks.
 - Use double backticks for struct member descriptions.
 - Fix a couple of build warnings by guarding pointers (*) with double
   backticks .e.g  ``*ptr``.
 - Add vfs to Documentation/filesystems/index.rst

The member descriptions paragraph indentation was not touched.  It is
not pretty but these do not cause build warnings.  These descriptions
all need updating anyways so leave it as it is for now.

Signed-off-by: Tobin C. Harding <tobin@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>