zram_drv: update for backing dev info changes

A previous commit made the bdi embedded in the request queue
a pointer, but neglected to fixup zram. Fix it up.

Fixes: dc3b17cc

 ("block: Use pointer to backing_dev_info from request_queue")
Reported-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Jens Axboe <axboe@fb.com>