nvme: simplify nvme_setup_prps calling convention

Pass back a true/false value instead of the length which needs a compare
with the bytes in the request and drop the pointless gfp_t argument.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>