xprtrdma: Update the GFP flags used in xprt_rdma_allocate()

Reflect the more conservative approach used in the socket transport's
version of this transport method. An RPC buffer allocation should
avoid forcing not just FS activity, but any I/O.

In particular, two recent changes missed updating xprtrdma:

 - Commit c6c8fe79a8 ("net, sunrpc: suppress allocation warning ...")
 - Commit a564b8f039 ("nfs: enable swap on NFS")

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

net/sunrpc/xprtrdma/transport.c

@@ -463,13 +463,16 @@ xprt_rdma_allocate(struct rpc_task *task, size_t size)
 	struct rpcrdma_regbuf *rb;
 	struct rpcrdma_req *req;
 	size_t min_size;
-	gfp_t flags = task->tk_flags & RPC_TASK_SWAPPER ?
-						GFP_ATOMIC : GFP_NOFS;
+	gfp_t flags;
 
 	req = rpcrdma_buffer_get(&r_xprt->rx_buf);
 	if (req == NULL)
 		return NULL;
 
+	flags = GFP_NOIO | __GFP_NOWARN;
+	if (RPC_IS_SWAPPER(task))
+		flags = __GFP_MEMALLOC | GFP_NOWAIT | __GFP_NOWARN;
+
 	if (req->rl_rdmabuf == NULL)
 		goto out_rdmabuf;
 	if (req->rl_sendbuf == NULL)