doc: filesystems: fix bad references to nonexistent ext4.rst file

The ext4.rst file does not exist anymore. This patch changes all references
to point to the whole ext4 directory.

Fixes: d3091215 ("docs: move ext4 administrative docs to admin-guide/")
Signed-off-by: Otto Sabart <ottosabart@seberm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>