btrfs: sink writepage parameter to extent_write_cache_pages

 * write_cache_pages - walk the list of dirty pages of the given address space and write all of them.

 * @mapping: address space structure to write

 * @wbc: subtract the number of written pages from *@wbc->nr_to_write

 * @writepage: function called for each page

 * @data: data passed to writepage function

 * @data: data passed to __extent_writepage function

 *

 * If a page is already under I/O, write_cache_pages() skips it, even

 * if it's dirty.  This is desirable behaviour for memory-cleaning writeback,

 */

static int extent_write_cache_pages(struct address_space *mapping,

			     struct writeback_control *wbc,

			     writepage_t writepage, void *data)

			     void *data)

{

	struct inode *inode = mapping->host;

	int ret = 0;

				continue;

			}

			ret = (*writepage)(page, wbc, data);

			ret = __extent_writepage(page, wbc, data);

			if (unlikely(ret == AOP_WRITEPAGE_ACTIVATE)) {

				unlock_page(page);

		.sync_io = wbc->sync_mode == WB_SYNC_ALL,

	};

	ret = extent_write_cache_pages(mapping, wbc, __extent_writepage, &epd);

	ret = extent_write_cache_pages(mapping, wbc, &epd);

	flush_write_bio(&epd);

	return ret;

}