NFSv4: Fix up another delegation related race

When we can update_open_stateid(), we need to be certain that we don't
race with a delegation return. While we could do this by grabbing the
nfs_client->cl_lock, a dedicated spin lock in the delegation structure
will scale better.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/delegation.h

@@ -22,6 +22,7 @@ struct nfs_delegation {
 	long flags;
 	loff_t maxsize;
 	__u64 change_attr;
+	spinlock_t lock;
 	struct rcu_head rcu;
 };