NFS: Replace flush_scheduled_work with cancel_work_sync() and friends

This will avoid deadlocks of the form:

stack backtrace:
 [<c0104fda>] show_trace_log_lvl+0x1a/0x30
 [<c0105c02>] show_trace+0x12/0x20
 [<c0105d15>] dump_stack+0x15/0x20
 [<c013ee42>] __lock_acquire+0xc22/0x1030
 [<c013f2b1>] lock_acquire+0x61/0x80
 [<c012edd9>] flush_workqueue+0x49/0x70
 [<c012ee0d>] flush_scheduled_work+0xd/0x10
 [<dcf55c0c>] nfs_release_automount_timer+0x2c/0x30 [nfs]
 [<dcf45d8e>] nfs_free_server+0x9e/0xd0 [nfs]
 [<dcf4e626>] nfs_kill_super+0x16/0x20 [nfs]
 [<c017b38d>] deactivate_super+0x7d/0xa0
 [<c018f94b>] mntput_no_expire+0x4b/0x80
 [<c018fd94>] expire_mount_list+0xe4/0x140
 [<c0191219>] mark_mounts_for_expiry+0x99/0xb0
 [<dcf55d1d>] nfs_expire_automounts+0xd/0x40 [nfs]
 [<c012e61b>] run_workqueue+0x12b/0x1e0
 [<c012f05b>] worker_thread+0x9b/0x100
 [<c0131c72>] kthread+0x42/0x70
 [<c0104c0f>] kernel_thread_helper+0x7/0x18
 =======================
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/namespace.c

@@ -175,10 +175,8 @@ static void nfs_expire_automounts(struct work_struct *work)
 
 void nfs_release_automount_timer(void)
 {
-	if (list_empty(&nfs_automount_list)) {
-		cancel_delayed_work(&nfs_automount_task);
-		flush_scheduled_work();
-	}
+	if (list_empty(&nfs_automount_list))
+		cancel_delayed_work_sync(&nfs_automount_task);
 }
 
 /*

fs/nfs/nfs4renewd.c

@@ -127,16 +127,15 @@ nfs4_schedule_state_renewal(struct nfs_client *clp)
 void
 nfs4_renewd_prepare_shutdown(struct nfs_server *server)
 {
-	flush_scheduled_work();
+	cancel_delayed_work(&server->nfs_client->cl_renewd);
 }
 
 void
 nfs4_kill_renewd(struct nfs_client *clp)
 {
 	down_read(&clp->cl_sem);
-	cancel_delayed_work(&clp->cl_renewd);
+	cancel_delayed_work_sync(&clp->cl_renewd);
 	up_read(&clp->cl_sem);
-	flush_scheduled_work();
 }
 
 /*