Even with CACHEFILES_DEAD set, we can still read the requests, so in the
following concurrency the request may be used after it has been freed:

     mount  |   daemon_thread1    |    daemon_thread2
------------------------------------------------------------
 cachefiles_ondemand_init_object
  cachefiles_ondemand_send_req
   REQ_A = kzalloc(sizeof(*req) + data_len)
   wait_for_completion(&REQ_A->done)
            cachefiles_daemon_read
             cachefiles_ondemand_daemon_read
                                  // close dev fd
                                  cachefiles_flush_reqs
                                   complete(&REQ_A->done)
   kfree(REQ_A)
              xa_lock(&cache->reqs);
              cachefiles_ondemand_select_req
                req->msg.opcode != CACHEFILES_OP_READ
                // req use-after-free !!!
              xa_unlock(&cache->reqs);
                                   xa_destroy(&cache->reqs)

Hence remove requests from cache->reqs when flushing them to avoid
accessing freed requests.

Fixes: c8383054 ("cachefiles: notify the user daemon when looking up cookie")
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Link: https://lore.kernel.org/r/20240522114308.2402121-3-libaokun@huaweicloud.comAcked-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Jia Zhu <zhujia.zj@bytedance.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>