NFSv4: Fix a slot leak in nfs40_sequence_done

The check for whether or not we sent an RPC call in nfs40_sequence_done
is insufficient to decide whether or not we are holding a session slot,
and thus should not be used to decide when to free that slot.

This patch replaces the RPC_WAS_SENT() test with the correct test for
whether or not slot == NULL.

Cc: Chuck Lever <chuck.lever@oracle.com>
Cc: stable@vger.kernel.org # 3.12+
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/nfs4proc.c

@@ -539,7 +539,7 @@ static int nfs40_sequence_done(struct rpc_task *task,
 	struct nfs4_slot *slot = res->sr_slot;
 	struct nfs4_slot_table *tbl;
 
-	if (!RPC_WAS_SENT(task))
+	if (slot == NULL)
 		goto out;
 
 	tbl = slot->table;