blktrace: Add blktrace ioctls to SCSI generic devices

Since the SCSI layer uses the request queues from the block layer, blktrace can
also be used to trace the requests to all SCSI devices (like SCSI tape drives),
not only disks. The only missing part is the ioctl interface to start and stop
tracing.

This patch adds the SETUP, START, STOP and TEARDOWN ioctls from blktrace to the
sg device files. With this change, blktrace can be used for SCSI devices like
for disks, e.g.: blktrace -d /dev/sg1 -o - | blkparse -i -

Signed-off-by: Christof Schmitt <christof.schmitt@de.ibm.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>