sg: pass the device name to blk_trace_setup

Fix a regression that passed a NULL device name to blk_trace_setup
accidentally.

Fixes: aebbb583 ("sg: do not allocate a gendisk")
Reported-by:  <syzbot+f74aa89114a236643919@syzkaller.appspotmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Link: https://lore.kernel.org/r/20210825075438.1883687-1-hch@lst.de


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