[CIFS] Fix oops on failed cifs mount (in kthread_stop)

If the cifs demultiplex thread wakes up and exits
(zeroing server->tsk) before kthread_stop is called, the
cifs_mount code could pass a null pointer to kthread_stop

Thanks to akpm, Dave Young and Shaggy for suggesting
earlier versions of this patch.

CC: akpm@linux-foundatior.org
Signed-off-by: Dave Young <hidave.darkstar@gmail.com>
Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>