SUNRPC: Generalize the RPC buffer allocation API

xprtrdma needs to allocate the Call and Reply buffers separately.
TBH, the reliance on using a single buffer for the pair of XDR
buffers is transport implementation-specific.

Transports that want to allocate separate Call and Reply buffers
will ignore the "size" argument anyway.  Don't bother passing it.

The buf_alloc method can't return two pointers. Instead, make the
method's return value an error code, and set the rq_buffer pointer
in the method itself.

This gives call_allocate an opportunity to terminate an RPC instead
of looping forever when a permanent problem occurs. If a request is
just bogus, or the transport is in a state where it can't allocate
resources for any request, there needs to be a way to kill the RPC
right there and not loop.

This immediately fixes a rare problem in the backchannel send path,
which loops if the server happens to send a CB request whose
call+reply size is larger than a page (which it shouldn't do yet).

One more issue: looks like xprt_inject_disconnect was incorrectly
placed in the failure path in call_allocate. It needs to be in the
success path, as it is for other call-sites.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprtsock.c

@@ -2533,23 +2533,28 @@ static void xs_tcp_print_stats(struct rpc_xprt *xprt, struct seq_file *seq)
  * we allocate pages instead doing a kmalloc like rpc_malloc is because we want
  * to use the server side send routines.
  */
-static void *bc_malloc(struct rpc_task *task, size_t size)
+static int bc_malloc(struct rpc_task *task)
 {
+	struct rpc_rqst *rqst = task->tk_rqstp;
+	size_t size = rqst->rq_callsize;
 	struct page *page;
 	struct rpc_buffer *buf;
 
-	WARN_ON_ONCE(size > PAGE_SIZE - sizeof(struct rpc_buffer));
-	if (size > PAGE_SIZE - sizeof(struct rpc_buffer))
-		return NULL;
+	if (size > PAGE_SIZE - sizeof(struct rpc_buffer)) {
+		WARN_ONCE(1, "xprtsock: large bc buffer request (size %zu)\n",
+			  size);
+		return -EINVAL;
+	}
 
 	page = alloc_page(GFP_KERNEL);
 	if (!page)
-		return NULL;
+		return -ENOMEM;
 
 	buf = page_address(page);
 	buf->len = PAGE_SIZE;
 
-	return buf->data;
+	rqst->rq_buffer = buf->data;
+	return 0;
 }
 
 /*