NVMe: Clear QUEUE_FLAG_STACKABLE

The nvme namespace request_queue's flags are initialized to
QUEUE_FLAG_DEFAULT, which currently sets QUEUE_FLAG_STACKABLE. The
device-mapper indicates this flag means the block driver is requset
based, though this driver is bio-based and problems will occur if an nvme
namespace is used with a request based dm device. This patch clears the
stackable flag.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>