CVE-2023-53366 block: be a bit more careful in checking for NULL bdev while polling

In the Linux kernel, the following vulnerability has been resolved: block: be a bit more careful in checking for NULL bdev while polling Wei reports a crash with an application using polled IO: PGD 14265e067 P4D 14265e067 PUD 47ec50067 PMD 0 Oops: 0000 1 SMP CPU: 0 PID: 21915 Comm: iocore0 Kdump:...