NFS: Fix use of cancel_delayed_work_sync in nfs_release_automount_timer

Doh! We can't use cancel_delayed_work_sync because we may have been called
from an unmount that was being performed by nfs_automount_task.
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/namespace.c

@@ -176,7 +176,7 @@ static void nfs_expire_automounts(struct work_struct *work)
 void nfs_release_automount_timer(void)
 {
 	if (list_empty(&nfs_automount_list))
-		cancel_delayed_work_sync(&nfs_automount_task);
+		cancel_delayed_work(&nfs_automount_task);
 }
 
 /*