bcache: fix unmatched generic_end_io_acct() & generic_start_io_acct()

The function cached_dev_make_request() and flash_dev_make_request() call
generic_start_io_acct() with (struct bcache_device)->disk when they start a
closure. Then the function bio_complete() calls generic_end_io_acct() with
(struct search)->orig_bio->bi_disk when the closure has done.
Since the `bi_disk` is not the bcache device, the generic_end_io_acct() is
called with a wrong device queue.

It causes the "inflight" (in struct hd_struct) counter keep increasing
without decreasing.

This patch fix the problem by calling generic_end_io_acct() with
(struct bcache_device)->disk.

Signed-off-by: Zhai Zhaoxuan <kxuanobj@gmail.com>
Reviewed-by: Michael Lyle <mlyle@lyle.org>
Reviewed-by: Coly Li <colyli@suse.de>
Reviewed-by: Tang Junhui <tang.junhui@zte.com.cn>
Signed-off-by: Jens Axboe <axboe@kernel.dk>