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: NOtto Sabart <ottosabart@seberm.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>