kernel: md/raid5-cache: fix a deadlock in r5l_exit_log()

A deadlock vulnerability was found in the md RAID5 cache r5l subsystem in the Linux kernel. In r5lexitlog, flushwork is called while holding reconfigmutex, which waits for disablewritebackwork to complete. However, r5cdisablewritebackasync calls waitevent which requires conf-log to be NULL, but...