devtmpfs: fix the dangling pointer of global devtmpfsd thread

When the devtmpfs fails to mount, a dangling pointer still remains in
global. Specifically, the err variable is passed by a pointer to the
devtmpfsd. When the devtmpfsd exits, it sets the error and completes the
setup_done. In this situation, the thread pointer is not set to null.
After the devtmpfsd exited, the devtmpfs can wakes up the destroyed
devtmpfsd thread by wake_up_process if a device change event comes.

Signed-off-by: Yangxi Xiang <xyangxi5@gmail.com>
Link: https://lore.kernel.org/r/20220627120409.11174-1-xyangxi5@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>