xprtrdma: Increase maximum number of backchannel requests

Reflects the change introduced in commit 067c4696 ("NFSv4.1:
Bump the default callback session slot count to 16").
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprtrdma/xprt_rdma.h

@@ -103,10 +103,12 @@ struct rpcrdma_ep {
 
 /* Pre-allocate extra Work Requests for handling backward receives
  * and sends. This is a fixed value because the Work Queues are
- * allocated when the forward channel is set up.
+ * allocated when the forward channel is set up, long before the
+ * backchannel is provisioned. This value is two times
+ * NFS4_DEF_CB_SLOT_TABLE_SIZE.
  */
 #if defined(CONFIG_SUNRPC_BACKCHANNEL)
-#define RPCRDMA_BACKWARD_WRS		(8)
+#define RPCRDMA_BACKWARD_WRS (32)
 #else
 #define RPCRDMA_BACKWARD_WRS (0)
 #endif