raid5-cache: simplify r5l_move_io_unit_list

It's only used for one kind of move, so make that explicit.  Also clean
up the code a bit by using list_for_each_safe.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Shaohua Li <shli@fb.com>
Signed-off-by: NeilBrown <neilb@suse.com>