9p: switch ->writepage() to direct use of p9_client_write()

static int v9fs_vfs_writepage_locked(struct page *page)

{

	char *buffer;

	int retval, len;

	loff_t offset, size;

	mm_segment_t old_fs;

	struct v9fs_inode *v9inode;

	struct inode *inode = page->mapping->host;

	struct v9fs_inode *v9inode = V9FS_I(inode);

	loff_t size = i_size_read(inode);

	struct iov_iter from;

	struct bio_vec bvec;

	int err, len;

	v9inode = V9FS_I(inode);

	size = i_size_read(inode);

	if (page->index == size >> PAGE_CACHE_SHIFT)

		len = size & ~PAGE_CACHE_MASK;

	else

		len = PAGE_CACHE_SIZE;

	set_page_writeback(page);

	buffer = kmap(page);

	offset = page_offset(page);

	bvec.bv_page = page;

	bvec.bv_offset = 0;

	bvec.bv_len = len;

	iov_iter_bvec(&from, ITER_BVEC | WRITE, &bvec, 1, len);

	old_fs = get_fs();

	set_fs(get_ds());

	/* We should have writeback_fid always set */

	BUG_ON(!v9inode->writeback_fid);

	retval = v9fs_file_write_internal(inode,

					  v9inode->writeback_fid,

					  (__force const char __user *)buffer,

					  len, &offset, 0);

	if (retval > 0)

		retval = 0;

	set_page_writeback(page);

	p9_client_write(v9inode->writeback_fid, page_offset(page), &from, &err);

	set_fs(old_fs);

	kunmap(page);

	end_page_writeback(page);

	return retval;

	return err;

}

static int v9fs_vfs_writepage(struct page *page, struct writeback_control *wbc)