[SCSI] scsi_debug: add max_queue + no_uld parameters

While testing the midlevel q_at_head and q_at_tail
patch for sg and the block SG_IO ioctl I found it
useful to reduce the queuing within the scsi_debug
driver. The reason is that the midlevel queue only
comes into play when the corresponding LLD queue
is full.

It is also useful when testing to be confident that
your program is the only thing issuing commands
to the (virtual) scsi_debug device. The no_uld=1
parameter will stop a scsi_debug virtual disk
appearing as /dev/sd* .

Changelog:
   - add max_queue parameter to reduce the number
     of queued commands the driver will accept.
     This parameter can be changed after the driver
     is loaded.
   - add no_uld parameter that restricts scsi_debug's
     virtual devices to the sg and bsg drivers
   - correct stale url

Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>