nvmet-passthru: Reject commands with non-sgl flags set

Any command with a non-SGL flag set (like fuse flags) should be
rejected.

Fixes: c1fef73f

 ("nvmet: add passthru code to process commands")
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>