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 <[e-mail]>

fs/nfs/namespace.c

···
 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);
 }
 
 /*