NVMe: Fix SG_IO status values

We've only been setting the sg_io_hdr status values on SCSI commands
that require an nvme command to complete the translation. The fields
in the struct are output parameters, so we have to set them, otherwise
user space will see whatever was in memory from before. In the case of
compat SG_IO, this would reveal kernel memory. This fixes the issue by
initializing the sg_io_hdr with successful status.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Acked-by: Vishal Verma <vishal.l.verma@linux.intel.com>
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>