bcache: fix NULL pointer reference in cached_dev_detach_finish

bcache: fix NULL pointer reference in cached_dev_detach_finish

Commit 0259d4498ba4 ("bcache: move calc_cached_dev_sectors to proper
place on backing device detach") tries to fix calc_cached_dev_sectors
when bcache device detaches, but now we have:

cached_dev_detach_finish
    ...
    bcache_device_detach(&dc->disk);
        ...
        closure_put(&d->c->caching);
        d->c = NULL; [*explicitly set dc->disk.c to NULL*]
    list_move(&dc->list, &uncached_devices);
    calc_cached_dev_sectors(dc->disk.c); [*passing a NULL pointer*]
    ...

Upper codeflows shows how bug happens, this patch fix the problem by
caching dc->disk.c beforehand, and cache_set won't be freed under us
because c->caching closure at least holds a reference count and closure
callback __cache_set_unregister only being called by bch_cache_set_stop
which using closure_queue(&c->caching), that means c->caching closure
callback for destroying cache_set won't be trigger by previous
closure_put(&d->c->caching).
So at this stage(while cached_dev_detach_finish is calling) it's safe to
access cache_set dc->disk.c.

Fixes: 0259d4498ba4 ("bcache: move calc_cached_dev_sectors to proper place on backing device detach")
Signed-off-by: Lin Feng <linf@wangsu.com>
Signed-off-by: Coly Li <colyli@suse.de>
Link: https://lore.kernel.org/r/20211112053629.3437-2-colyli@suse.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>