NVMe: Don't allow unsupported flags

The command flags can change the meaning of other fields in the command
that the driver is not prepared to handle. Specifically, the user could
passthrough an SGL flag, causing the controller to misinterpret the PRP
list the driver created, potentially corrupting memory or data.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Jon Derrick <jonathan.derrick@intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Jens Axboe <axboe@fb.com>