ext4: docs: switch away from list-table

Commit 3a6541e9

 (Add documentation about the orphan file feature) added
a new document on orphan files, which is great.  But the use of
"list-table" results in documents that are absolutely unreadable in their
plain-text form.  Switch this file to the regular RST table format instead;
the rendered (HTML) output is identical.

Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20210902220854.198850-2-corbet@lwn.net
Signed-off-by: Jonathan Corbet <corbet@lwn.net>