target: stop using blk_make_request

Using blk_rq_append_bio allows to append the bios to the request
directly instead of having to build up a list first, and also
allows to have a single code path for requests with or without
data attached to them.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>