Now that we don't abuse the cmd field in struct request for nvme command
passthrough this function needs to be converted to the proper accessor
as well.

Fixes: d49187e9 ("nvme: introduce struct nvme_request")
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NMax Gurtovoy <maxg@mellanox.com>