NFSv4: Don't remove the delegation from the super_list more than once

Add a check to ensure that we haven't already removed the delegation
from the inode after we take all the relevant locks.
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/delegation.c

@@ -298,6 +298,10 @@ nfs_detach_delegation_locked(struct nfs_inode *nfsi,
 		return NULL;
 
 	spin_lock(&delegation->lock);
+	if (!delegation->inode) {
+		spin_unlock(&delegation->lock);
+		return NULL;
+	}
 	set_bit(NFS_DELEGATION_RETURNING, &delegation->flags);
 	list_del_rcu(&delegation->super_list);
 	delegation->inode = NULL;