swiotlb: move slot allocation explanation comment where it belongs

Move the comment down in front of the loop that actually sets the list
member of struct io_tlb_slot to zero.

Fixes: 26a7e094 ("swiotlb: refactor swiotlb_tbl_map_single")
Signed-off-by: Petr Tesarik <petr.tesarik.ext@huawei.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>