SUNRPC: Provide a callback to allow free pages allocated during xdr encoding

For privacy, we need to allocate pages to store the encrypted data (passed
 in pages can't be used without the risk of corrupting data in the page cache).
 So we need a way to free that memory after the request has been transmitted.

 Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
 Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

include/linux/sunrpc/xprt.h

@@ -73,7 +73,10 @@ struct rpc_rqst {
 	int			rq_cong;	/* has incremented xprt->cong */
 	int			rq_received;	/* receive completed */
 	u32			rq_seqno;	/* gss seq no. used on req. */
-
+	int			rq_enc_pages_num;
+	struct page		**rq_enc_pages;	/* scratch pages for use by
+						   gss privacy code */
+	void (*rq_release_snd_buf)(struct rpc_rqst *); /* release rq_enc_pages */
 	struct list_head	rq_list;
 
 	struct xdr_buf		rq_private_buf;		/* The receive buffer

net/sunrpc/xprt.c

@@ -839,6 +839,7 @@ static void xprt_request_init(struct rpc_task *task, struct rpc_xprt *xprt)
 	req->rq_task	= task;
 	req->rq_xprt    = xprt;
 	req->rq_xid     = xprt_alloc_xid(xprt);
+	req->rq_release_snd_buf = NULL;
 	dprintk("RPC: %4d reserved req %p xid %08x\n", task->tk_pid,
 			req, ntohl(req->rq_xid));
 }
@@ -867,6 +868,8 @@ void xprt_release(struct rpc_task *task)
 				xprt->last_used + xprt->idle_timeout);
 	spin_unlock_bh(&xprt->transport_lock);
 	task->tk_rqstp = NULL;
+	if (req->rq_release_snd_buf)
+		req->rq_release_snd_buf(req);
 	memset(req, 0, sizeof(*req));	/* mark unused */
 
 	dprintk("RPC: %4d release request %p\n", task->tk_pid, req);