ext2: Convert ext2_make_empty() to use a folio

Remove two hidden calls to compound_head() by using the folio API.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Message-Id: <20230921200746.3303942-9-willy@infradead.org>