->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_releasepage(struct page *page, gfp_t unused_gfp_flags)
{
	/*
	 * An attempt to release a dirty page without budgeting for it - should
	 * not happen.
	 */
	if (PageWriteback(page))
		return 0;
	ubifs_assert(PagePrivate(page));
	ubifs_assert(0);
	ClearPagePrivate(page);
	ClearPageChecked(page);
	return 1;
}

static int ubifs_releasepage(struct page *page, gfp_t unused_gfp_flags)
{
	/*
	 * An attempt to release a dirty page without budgeting for it - should
	 * not happen.
	 */
	if (PageWriteback(page))
		return 0;
	ubifs_assert(PagePrivate(page));
	ubifs_assert(0);
	ClearPagePrivate(page);
	ClearPageChecked(page);
	return 1;
}
