nfsd: do not clear rpc_resp in nfsd4_cb_done_sequence

This is incorrect when a callback is has to be restarted, in which case
the XDR decoding of the second iteration will see a NULL cb argument.

[hch: updated description]
Signed-off-by: NBenny Halevy <bhalevy@panasas.com>
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4callback.c

@@ -847,9 +847,6 @@ static void nfsd4_cb_done(struct rpc_task *task, void *calldata)
 		rpc_wake_up_next(&clp->cl_cb_waitq);
 		dprintk("%s: freed slot, new seqid=%d\n", __func__,
 			clp->cl_cb_session->se_cb_seq_nr);
-
-		/* We're done looking into the sequence information */
-		task->tk_msg.rpc_resp = NULL;
 	}
 }