block: fix a crash caused by wrong API

part_stat_show takes a part device not a disk, so we should use
part_to_disk.

Fixes: d62e26b3

("block: pass in queue to inflight accounting")
Cc: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Omar Sandoval <osandov@fb.com>
Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>