ext2: Convert ext2_add_link() to use a folio

Remove five hidden calls to compound_head() and fix a couple of
places that assumed PAGE_SIZE.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Message-Id: <20230921200746.3303942-5-willy@infradead.org>