[SCSI] sg: fix device number in blktrace data

Hi,

we have run into an issue with blktrace being started for sg devices.
Please apply.

Thanks,
Martin

From: Martin Peschke <mpeschke@linux.vnet.ibm.com>

The device number denoting a generic SCSI devices (sg) in a blktrace
trace is broken; major and minor are always 0. It looks like
sdp->device->sdev_gendev.devt is not initialized properly.
The fix below uses other data to make up a valid device number,
similar to the way an sg device number is generated for sysfs output.

Reported-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Signed-off-by: Martin Peschke <mpeschke@linux.vnet.ibm.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>