ext2: Convert ext2_readdir to use a folio

Saves a hidden call to compound_head().

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Message-Id: <20230921200746.3303942-4-willy@infradead.org>