bcache: fix cached_dev.sb_bio use-after-free and crash

commit fec114a98b8735ee89c75216c45a78e28be0f128 upstream.

In our production environment, we have received multiple crash reports
regarding libceph, which have caught our attention:

```
[6888366.280350] Call Trace:
[6888366.280452]  blk_update_request+0x14e/0x370
[6888366.280561]  blk_mq_end_request+0x1a/0x130
[6888366.280671]  rbd_img_handle_request+0x1a0/0x1b0 [rbd]
[6888366.280792]  rbd_obj_handle_request+0x32/0x40 [rbd]
[6888366.280903]  __complete_request+0x22/0x70 [libceph]
[6888366.281032]  osd_dispatch+0x15e/0xb40 [libceph]
[6888366.281164]  ? inet_recvmsg+0x5b/0xd0
[6888366.281272]  ? ceph_tcp_recvmsg+0x6f/0xa0 [libceph]
[6888366.281405]  ceph_con_process_message+0x79/0x140 [libceph]
[6888366.281534]  ceph_con_v1_try_read+0x5d7/0xf30 [libceph]
[6888366.281661]  ceph_con_workfn+0x329/0x680 [libceph]
```

After analyzing the coredump file, we found that the address of
dc->sb_bio has been freed. We know that cached_dev is only freed when it
is stopped.

Since sb_bio is a part of struct cached_dev, rather than an alloc every
time.  If the device is stopped while writing to the superblock, the
released address will be accessed at endio.

This patch hopes to wait for sb_write to complete in cached_dev_free.

It should be noted that we analyzed the cause of the problem, then tell
all details to the QWEN and adopted the modifications it made.

BUG=b/506404196
TEST=presubmit
RELEASE_NOTE=Fixed CVE-2026-31580 in the Linux kernel.

Change-Id: I65d7c2ad1aff23d5845ef2df8e0d3ba240a812c8
Signed-off-by: Mingzhe Zou <mingzhe.zou@easystack.cn>
Fixes: cafe563591446 ("bcache: A block layer cache")
Cc: stable@vger.kernel.org # 3.10+
Signed-off-by: Coly Li <colyli@fnnas.com>
Link: https://patch.msgid.link/20260322134102.480107-1-colyli@fnnas.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Angel Adetula <angeladetula@google.com>
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/148007
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>