scsi: sg: only check for dxfer_len greater than 256M

Don't make any assumptions on the sg_io_hdr_t::dxfer_direction or the
sg_io_hdr_t::dxferp in order to determine if it is a valid request. The
only way we can check for bad requests is by checking if the length
exceeds 256M.

Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Fixes: 28676d86 (scsi: sg: check for valid direction before starting the
request)
Reported-by: Jason L Tibbitts III <tibbs@math.uh.edu>
Tested-by: Jason L Tibbitts III <tibbs@math.uh.edu>
Suggested-by: Doug Gilbert <dgilbert@interlog.com>
Cc: Doug Gilbert <dgilbert@interlog.com>
Cc: <stable@vger.kernel.org>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>