cifs: No need to send SIGKILL to demux_thread during umount

There is no need to explicitly send SIGKILL to cifs_demultiplex_thread
as it is calling module_put_and_exit to exit cleanly.

socket sk_rcvtimeo is set to 7 HZ so the thread will wake up in 7 seconds and
clean itself.

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Ashish Sangwan <a.sangwan@samsung.com>
Acked-by: Jeff Layton <jlayton@samba.org>
Signed-off-by: Steve French <smfrench@gmail.com>