ext2: Add ext2_get_folio()

Convert ext2_get_page() into ext2_get_folio() and keep the original
function around as a temporary wrapper.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Message-Id: <20230921200746.3303942-3-willy@infradead.org>