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

In the Linux kernel, the following vulnerability has been resolved:

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

Commit b13015af94cf ("md/raid5-cache: Clear conf->log after finishing

work") introduce a new problem:

// caller hold reconfig_mutex

r5l_exit_log

flush_work(&log->disable_writeback_work)

r5c_disable_writeback_async

wait_event

/*

* conf->log is not NULL, and mddev_trylock()

* will fail, wait_event() can never pass.

*/

conf->log = NULL

Fix this problem by setting 'config->log' to NULL before wake_up() as it

used to be, so that wait_event() from r5c_disable_writeback_async() can

exist. In the meantime, move forward md_unregister_thread() so that

null-ptr-deref this commit fixed can still be fixed.