->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>
xfs_vm_page_mkwrite(
	struct vm_area_struct	*vma,
	struct vm_fault		*vmf)
{
	return block_page_mkwrite(vma, vmf, xfs_get_blocks);
}

xfs_vm_page_mkwrite(
	struct vm_area_struct	*vma,
	struct vm_fault		*vmf)
{
	return block_page_mkwrite(vma, vmf, xfs_get_blocks);
}
