Documentation: filesystems: update filename extensions

Update references to most txt files to rst files.
Update one reference to an md file to a rst file.
Update one file path to its current location.

Signed-off-by: Randy Dunlap <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: [e-mail]
Cc: Alexander Viro <[e-mail]>
Cc: Christian Brauner <[e-mail]>
Cc: Jan Kara <[e-mail]>
Cc: [e-mail]
Cc: Ian Kent <[e-mail]>
Cc: [e-mail]
Cc: Alexander Aring <[e-mail]>
Cc: David Teigland <[e-mail]>
Cc: [e-mail]
Cc: Eric Biggers <[e-mail]>
Cc: Theodore Y. Ts'o <[e-mail]>
Cc: [e-mail]
Cc: Mark Fasheh <[e-mail]>
Cc: Joel Becker <[e-mail]>
Cc: Joseph Qi <[e-mail]>
Cc: [e-mail]
Reviewed-by: Christian Brauner (Microsoft) <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/filesystems/autofs.rst

···
 It requires CAP_SYS_ADMIN for access.
 
 The 'ioctl's that can be used on this device are described in a separate
-document `autofs-mount-control.txt`, and are summarised briefly here.
+document `autofs-mount-control.rst`, and are summarised briefly here.
 Each ioctl is passed a pointer to an `autofs_dev_ioctl` structure::
 
         struct autofs_dev_ioctl {

Documentation/filesystems/dlmfs.rst

···
 Usage
 =====
 
-If you're just interested in OCFS2, then please see ocfs2.txt. The
+If you're just interested in OCFS2, then please see ocfs2.rst. The
 rest of this document will be geared towards those who want to use
 dlmfs for easy to setup and easy to use clustered locking in
 userspace.

Documentation/filesystems/fsverity.rst

···
 code is needed to support fs-verity.
 
 fs-verity is similar to `dm-verity
-<https://www.kernel.org/doc/Documentation/device-mapper/verity.txt>`_
+<https://www.kernel.org/doc/Documentation/admin-guide/device-mapper/verity.rst>`_
 but works on files rather than block devices.  On regular files on
 filesystems supporting fs-verity, userspace can execute an ioctl that
 causes the filesystem to build a Merkle tree for the file and persist

Documentation/filesystems/path-lookup.rst

···
 Automount points are locations in the filesystem where an attempt to
 lookup a name can trigger changes to how that lookup should be
 handled, in particular by mounting a filesystem there.  These are
-covered in greater detail in autofs.txt in the Linux documentation
+covered in greater detail in autofs.rst in the Linux documentation
 tree, but a few notes specifically related to path lookup are in order
 here.
 

Documentation/filesystems/path-lookup.txt

···
 
 2. http://lse.sourceforge.net/locking/dcache/dcache.html
 
-3. path-lookup.md in this directory.
+3. path-lookup.rst in this directory.

Documentation/filesystems/ramfs-rootfs-initramfs.rst

···
 2) The cpio archive format chosen by the kernel is simpler and cleaner (and
    thus easier to create and parse) than any of the (literally dozens of)
    various tar archive formats.  The complete initramfs archive format is
-   explained in buffer-format.txt, created in usr/gen_init_cpio.c, and
+   explained in buffer-format.rst, created in usr/gen_init_cpio.c, and
    extracted in init/initramfs.c.  All three together come to less than 26k
    total of human-readable text.