cachefiles: wait for ondemand_object_worker to finish when dropping object
In the Linux kernel, the following vulnerability has been resolved:

cachefiles: wait for ondemand_object_worker to finish when dropping object

When queuing ondemand_object_worker() to re-open the object,
cachefiles_object is not pinned. The cachefiles_object may be freed when
the pending read request is completed intentionally and the related
erofs is umounted. If ondemand_object_worker() runs after the object is
freed, it will incur use-after-free problem as shown below.

process A  processs B  process C  process D

cachefiles_ondemand_send_req()
// send a read req X
// wait for its completion

           // close ondemand fd
           cachefiles_ondemand_fd_release()
           // set object as CLOSE

                       cachefiles_ondemand_daemon_read()
                       // set object as REOPENING
                       queue_work(fscache_wq, &info->ondemand_work)

                                // close /dev/cachefiles
                                cachefiles_daemon_release
                                cachefiles_flush_reqs
                                complete(&req->done)

// read req X is completed
// umount the erofs fs
cachefiles_put_object()
// object will be freed
cachefiles_ondemand_deinit_obj_info()
kmem_cache_free(object)
                       // both info and object are freed
                       ondemand_object_worker()

When dropping an object, it is no longer necessary to reopen the object,
so use cancel_work_sync() to cancel or wait for ondemand_object_worker()
to finish.