Revert "cifs: No need to send SIGKILL to demux_thread during umount"

This reverts commit 52a36244.

Causes rmmod to fail for at least 7 seconds after unmount which
makes automated testing a little harder when reloading cifs.ko
between test runs.

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
CC: Jeff Layton <jlayton@primarydata.com>
Signed-off-by: Steve French <smfrench@gmail.com>