[CIFS] Switch cifsd to kthread_run from kernel_thread

cifsd was the only cifs thread that had not been switched to the newer
kthread interface

Signed-off-by: Igor Mammedov <niallain at gmail.com>
Signed-off-by: Wilhelm Meier <wilhelm.meier@fh-kl.de>
Signed-off-by: Steve French <sfrench@us.ibm.com>