cifs: fix a regression in cifs umount codepath

cifs: fix a regression in cifs umount codepath

backport of 469ee614aaa367d9cde01cbdd2027212f56c6cc6 upstream.

Several cifs patches were added to 2.6.27.8 to fix some races in the
mount/umount codepath. When this was done, a couple of prerequisite
patches were missed causing a minor regression.

When the last cifs mount to a server goes away, the kthread that manages
the socket is supposed to come down. The patches that went into 2.6.27.8
removed the kthread_stop calls that used to take down these threads, but
left the thread function expecting them. This made the thread stay up
even after the last mount was gone.

This patch should fix up this regression and also prevent a possible
race where a dead task could be signalled.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Cc: Suresh Jayaraman <sjayaraman@suse.de>
Acked-by: Steve French <smfrench@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>