->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>
ext4_unaligned_aio(struct inode *inode, struct iov_iter *from, loff_t pos)
{
	struct super_block *sb = inode->i_sb;
	int blockmask = sb->s_blocksize - 1;

	if (pos >= i_size_read(inode))
		return 0;

	if ((pos | iov_iter_alignment(from)) & blockmask)
		return 1;

	return 0;
}

ext4_unaligned_aio(struct inode *inode, struct iov_iter *from, loff_t pos)
{
	struct super_block *sb = inode->i_sb;
	int blockmask = sb->s_blocksize - 1;

	if (pos >= i_size_read(inode))
		return 0;

	if ((pos | iov_iter_alignment(from)) & blockmask)
		return 1;

	return 0;
}
