NFSv4: Handle the condition NFS4ERR_FILE_OPEN

Retry a few times before we give up: the error is usually due to ordering
issues with asynchronous RPC calls.

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

fs/nfs/nfs4proc.c

@@ -2891,6 +2891,7 @@ int nfs4_handle_exception(const struct nfs_server *server, int errorcode, struct
 			if (ret == 0)
 				exception->retry = 1;
 			break;
+		case -NFS4ERR_FILE_OPEN:
 		case -NFS4ERR_GRACE:
 		case -NFS4ERR_DELAY:
 			ret = nfs4_delay(server->client, &exception->timeout);