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 <[e-mail]>
Cc: [e-mail] # 3.12+
Signed-off-by: Trond Myklebust <[e-mail]>

fs/nfs/nfs4proc.c

···
 	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;