block: fix bounce_clone_bio for passthrough bios

Now that bio_alloc_bioset does not fall back to kmalloc for a NULL
bio_set, handle that case explicitly and simplify the calling
conventions.

Based on an earlier patch from Chaitanya Kulkarni.

Fixes: 3175199a ("block: split bio_kmalloc from bio_alloc_bioset")
Reported-by: Chaitanya Kulkarni <Chaitanya.Kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>