scsi/sg: don't grab scsi host module reference

In order to prevent request_queue to be freed before cleaning up
blktrace debugfs entries, commit db59133e

 ("scsi: sg: fix blktrace
debugfs entries leakage") use scsi_device_get(), however,
scsi_device_get() will also grab scsi module reference and scsi module
can't be removed.

It's reported that blktests can't unload scsi_debug after block/001:

blktests (master) # ./check block
block/001 (stress device hotplugging) [failed]
     +++ /root/blktests/results/nodev/block/001.out.bad 2023-06-19
      Running block/001
      Stressing sd
     +modprobe: FATAL: Module scsi_debug is in use.

Fix this problem by grabbing request_queue reference directly, so that
scsi host module can still be unloaded while request_queue will be
pinged by sg device.

Reported-by: Chaitanya Kulkarni <chaitanyak@nvidia.com>
Link: https://lore.kernel.org/all/1760da91-876d-fc9c-ab51-999a6f66ad50@nvidia.com/
Fixes: db59133e

 ("scsi: sg: fix blktrace debugfs entries leakage")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230621160111.1433521-1-yukuai1@huaweicloud.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>