->splice_write() via ->write_iter()

iter_file_splice_write() - a ->splice_write() instance that gathers the
pipe buffers, builds a bio_vec-based iov_iter covering those and feeds
it to ->write_iter().  A bunch of simple cases coverted to that...

[AV: fixed the braino spotted by Cyrill]

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
static int ubifs_set_page_dirty(struct page *page)
{
	int ret;

	ret = __set_page_dirty_nobuffers(page);
	/*
	 * An attempt to dirty a page without budgeting for it - should not
	 * happen.
	 */
	ubifs_assert(ret == 0);
	return ret;
}

static int ubifs_set_page_dirty(struct page *page)
{
	int ret;

	ret = __set_page_dirty_nobuffers(page);
	/*
	 * An attempt to dirty a page without budgeting for it - should not
	 * happen.
	 */
	ubifs_assert(ret == 0);
	return ret;
}
